Does Monday still look like a severe day?

A few days have come and gone since our last post about the potential severe weather event for the first day of June. So how are things looking now that the event is nearing? Well according to the water vapor imagery below, it's looking pretty good. See the swirl off the northern California Coast? That's the deep low-pressure system which will deliver the active weather as it moves toward the Inland Northwest. Why is this low important? Well for a good thunderstorm day we need three key ingredients: lift, instability, and moisture. The low will provide us with the first ingredient.

Animated water vapor imagery. Note the swirl off the Northern California coast. 

The models are very consistent in taking this low from its current position off the California Coast toward the Washington/Oregon border by afternoon and then toward the Washington/BC border by evening. This pattern resembles the typical negative tilted pattern we associate with severe weather in the Inland Northwest. The track of the low will ensure that the region will be subject to strong lifting potential beginning in southern Washington and central Idaho in the morning and spreading northward through the day. 

500 mb forecast with precipitable water forecast

Notice the shading in the 500 mb map above? That represents the precipitable water forecast. The greens, blues, and purples depict where the juiciest air will be located, Initially the pool of moisture, currently over far northern Oregon, will shift northward tonight and then become enhanced with even more moisture from the incoming low. How much moisture will we see? We forecast that using  a parameter termed precipitable water. Precipitable water is a figure used to represent how much water the atmosphere is holding. The precipitable water forecasts are expected to reach  values are forecast to exceed an inch over portions of the Inland Northwest by afternoon. How unusual is that? According to the graphic below, it would be placed in the 97.5-99th percentile for this time of year. So it will be far from typical. 

Precipitable Water anomaly

So with the two key pieces in place, what about the third, instability? Well it looks like that piece of the puzzle will be realized as well. If you recall our previous blog post, one of the ways to measure potential instability is looking at the lifted index values. If we see negative lifted index values, that indicates potential instability. So as early as 5 am, we begin to see some of this instability. Notice the yellow and orange shading over southeast Washington? This is enough instability to support thunderstorms even without the benefit of daytime heating. 

5am Monday Lifted Index Forecast

How about later in the day? Suffice it to say the instability will not be lacking. The entire region is expected to see negative lifted index values, with the best instability represented by the -6 to -8 values over extreme eastern Washington and north Idaho. That's about as good of a lifted index as you can expect to see over the region. 

5 pm Monday Lifted Index Forecast

So with all the pieces in place what would we expect to happen? We think there will be a band of showers and thunderstorms early in the morning spreading across the Washington/Oregon border, which should slowly work their way northward through the morning. How extensive this band will be remains rather uncertain. One of the weather models is forecasting the radar to look like the image below. Notice the nice cluster of showers and thunderstorms near the Blue Mountains in SE Washington. This coincides nicely with the good pocket of instability forecast by another model.  

7 am simulated radar image

By midday, this band is expected to drift farther north, but could lose some of its definition and strength.  

11 am simulated radar

However, the first band which moves through will further moisten and destabilize the atmosphere before the strong upper-level disturbance tracks toward our region. So what will the afternoon look like? This is far from certain, and there are as many answers as there are high-resolution models. Here's just one of the forecasts for 5 pm. 

5 pm simulated radar

That is a pretty impressive simulation with strong thunderstorms located across the northern portions of Washington as well as in the southern Idaho Panhandle. However recall that the best instability (or lifted index values) were located over the Idaho Panhandle. So we have better faith in the eastern portions of this radar simulation verifying. Another thing we can look at is an ensemble of simulated radar returns. The image below shows where the greatest risk will be for the biggest thunderstorms (chances of having a 40 dbz or stronger radar echo). The regions shaded in purple have the best chances (over 90%) followed closely by the reds (over 70%). This would highlight two areas. The Cascade crest and over the southeast corner of Washington and the southern Idaho Panhandle.

Ensemble chances for 40dbz or higher echoes

So what will the main risk of severe weather involve tomorrow? Based on the instability, the biggest risk looks to be large hail. If we look at the model soundings they are likely a little too moist to support widespread wind damage, however, we still expect to see some strong wind potential with a few of the storms. 

So what is the typical weather we experience with this type of upper-level pattern? We can utilize a fascinating tool produced by the Cooperative Institute for Precipitation Systems (CIPS) at Saint Louis University. They created a tool that makes an analog of the 15 closest weather patterns (since 1980) to what the forecast weather pattern is supposed to be. So below we see the forecast for tomorrow in red compared to the mean of the top 15 analogs (or pattern matches). This looks like a pretty good fit.

500 mb forecast for tomorrow (red) plotted against the 15 top weather analogs since 1980

So what weather was experienced on those days? Interestingly enough, quite a bit. Of those 15 days that matched Mondays expected weather, there were widespread severe reports. Most of them were related to hail, but a good sample of the reports were related to wind as well. Also, notice there were 3 tornado reports in the Inland Northwest. We do not expect to see tornadoes on Monday, as the wind patterns are not quite right to support violently rotating storms, however, they have occurred in this weather pattern before. 

Severe reports from the top 15 analog days to Monday's weather pattern

The other risk we see tomorrow will be for flash flooding. The storms which develop tomorrow will obviously contain copious amounts of moisture. And more importantly they could be slow movers. 

The risk of heavy rains will likely continue through Monday night before tapering off. Here's a look at the 24-hour precipitation forecast from 4 different weather models.

Precipitation forecast from 11am Mon-11am Tue

That's a lot of precipitation (purple amounts are above  0.75") and there is a fairly consistent message that the bulk of it will occur across the northern quarter of Washington, the Cascade Crest, and over the Idaho Panhandle. Since much of this will be attributed to thunderstorms, there is likely to be a high variability over short distances, with some areas likely to receive much heavier amounts. 

Stay tuned for updates to the forecast as this will likely be a very active weather pattern. 

Is it still looking wet for this Memorial Day Weekend?

So a couple days have passed since our last blog post about the weather outlook for the Inland Northwest for the upcoming Memorial Day weekend. Consequently, our confidence in what sort of weather to expect should be increasing. Recall that in the last post, there were suggestions that this could be a very wet weekend. In fact, perhaps the wettest Memorial Day weekend since 1997 and the second wettest on record (since 1970). So is that still the case?

Saturday 500 mb height

Above is the 500 mb map for Saturday from the GFS. This is a similar setup to the maps we explored earlier this week with a deep low centered over the Pacific Northwest. So is this GFS run an outlier or do other model agree with the solution?

4 different models showing 500 mb heights for Saturday (upper left model did not come in fully)

Here's is a look at 4 models compared to each other and they all show a low focused somewhere over the Pacific Northwest on Saturday. So where is this low on Sunday?

500 mb maps for Sunday

Again there is great agreement that there will be a low focused somewhere over the Pacific Northwest. The top 2 models take the low into southern Oregon, whereas the bottom two models drop the low into southern Oregon. Now, what the positioning of the low on Memorial Day?

500 mb maps for Monday

Again all the models keep the low over the region, however, the consensus is to drop the low south of the Washington/Oregon border.

So our confidence in this event is quite high. We are certain this low will form and move over portions of the Inland Northwest however how long it will remain over the area and where its precipitation  band will set up is the big question. Looking at the GFS for the Saturday and Sunday yields a significant northwest to southeast orientated band of precipitation.

48 hr precipitation amounts for Saturday-Sunday (latest model run )

This is quite similar to the previous model run we looked at a couple days ago(see below). Both model runs show a similarly oriented band of precipitation, however,  the latest version has the band a little farther to the west leaving places such as Sandpoint and Colville much drier than the previous forecast.

Rainfall forecast for Saturday and Sunday (Monday's model run)

Obviously wherever this band sets up will determine who gets a lot of rain and who doesn't. If we decide to consult another model solution it also shows a similar NW-SE band setting up, but this time it's displaced even further west. If this solution were to pan out even Spokane would miss out on the bulk of the rain as would the Silver Valley. 

Another Saturday-Sunday 48 hr precipitation forecast

So given these uncertainties it will be hard to pinpoint exactly where the band will take up residence. This is where ensemble model forecast can sometimes help to pinpoint the band (s) of heavy precipitation. So once again referring to the GEFS/NAEFS ensemble forecasts we see that it maintains a NW-SE oriented band of heavy precipitation as well. However the model has backed off from previous runs which suggested this would be a 99-99.9 percentile event for this time of year. Nonetheless the model still suggests this is potentially a 90-97 percentile event which is still significant. 

NAEFS/GEFS Ensemble 48 hr rainfall Sat-Sun

So what about the temperature forecast? We are holding onto our thoughts of hitting near 70° each day in Spokane this weekend, however if the band hits our area what will our temperature be? To say there is some uncertainty in this forecast would be an understatement. The 12z run of the GFS (dark blue line) has our high hitting the mid-70s that day. How about the 18z run of the GFS? A chilly mid-50s! That is about as uncertain as things can get. Its pretty much the same story for other parts of the Inland Northwest including Moses Lake (bottom meteogram)

Meteogram for Spokane

Meteogram for Moses Lake

This forecast remains a problematic one. Suffice it to say a good portion of the Inland Northwest is going to see a soggy beginning to the holiday weekend. Specifically where that will be, we still can't say with good confidence.  We expect the wettest day of the three-day weekend will be Saturday with a gradual improvement expected after that.

How is this Memorial Day Weekend looking weather-wise?

Last year at this time, we composed an extensive blog discussing the connotations associated with Memorial Day weekends in the Inland Northwest.  In many parts of the country, this weekend typically kicks off the summer season. However here in the Inland Northwest that really isn't the case, due to continued cool and unsettled weather. So is this year going to fit the weather mold, or will we see warm and dry weather?

Before we delve into this year's weather scenario, lets look at what the typical weather pattern is for this time of year. Looking at the mean 500 mb (18k ft MSL) maps below we see that the typical weather pattern is to see moist southwest flow pointed into the Pacific Northwest with a ridge setting up just downstream of the Continental Divide in western and central Montana. 

500 mb mean map 5/23-5/31

So what about the temperatures associated with this average pattern? Here's  the average temperatures for a couple locations as well as the average chance of precipitation:

So not bad. An average high around 70°F with rain expected on roughly 1 out of 4 days doesn't sound too terrible. But that's climatology and this weekend's weather could vary significantly from the norm. Why you ask? Well, let us take a look at the 500 mb pattern for this weekend to get an idea what sort of weather we might be dealing with. First off, here's a look at the weather for Saturday. 

GFS 500 mb heights for Saturday

This does not look promising if you enjoy plenty of sunshine and warmth. Note the closed low-pressure system (yellow L) centered over northwest Washington. Low-pressure systems like this are typically associated with cool and unsettled weather, especially if your are under or near the low. But this is only one model run. Surely there are other ideas out there in weather land. Well, here's a look at 4 distinct weather models for the same time period.

500 mb pattern from 4 different weather models for Saturday

Notice all these model runs place a low somewhere over the Pacific Northwest. That raises our confidence that the weather will be cool and unsettled over portions of the region, but maybe this unsettled weather will only last for a day. So let us take a look at the Sunday weather pattern.

500 mb pattern from 4 different weather models for Sunday

If the upper left model solution verifies (the GFS), the Inland Northwest could really be stricken with another cool and wet day. The other model solutions would hint at some modest improvements, but far from what we would deem summer like weather. How about Monday then?

500 mb pattern for Monday

The upper left solution (the GFS) still keeps the low parked over Washington while the others either drop it south of the region or weaken it. We shall see. But what if the GFS solution were to verify? What would our weather be like? If you love rain, well then you will love this forecast. Here's a look at the total precipitation forecast for Saturday and Sunday.

Rainfall forecast for Saturday and Sunday

The swath of red shading indicates rainfall amounts in excess of an inch. This includes Spokane, Kellogg, Coeur d'Alene, Pullman, Lewiston, Banks Lake, Lake Roosevelt, and Colville. If you look even closer there are a few locations which are expected to see in excess of 2 inches (shaded in tan)!
Now that is a lot of rain, regardless of the time of year. If the precipitation were to materialize, it would equate to our wettest Memorial Day weekend since 1997 and the second wettest on record (since 1970). So what is our confidence level in this precipitation panning out at this point?

Another tool we can look at in addition to just a few deterministic model runs is from an ensemble forecast approach. This is where you look at multiple model runs (initialized from the same model) and try to gather where the forecast confidence in the precipitation swath is greatest. Using this approach, the news isn't great either.

NAEFS/GEFS Ensemble precipitation amounts for the 48 hrs ending Sunday Afternoon

Without delving too deep into the specifics of the image above, the black lines depict the average precipitation amounts from the ensemble mean precipitation forecast while the green and blue shading (well any shading really) is bad if you like dry weather.  Notice the black lines hint at a swath of precipitation extending from southern British Columbia to the central Idaho Panhandle. The green and blue shading suggests the 48-hour precipitation amounts are in the 99-99.9% percentile for an early April-early July event. In other words, it is a 1 out of 100 or 1 out of a 1000 event for this particular time of year! There is even a small area of pale blue which hints that rain of this magnitude for this time of year has not been experienced since between 1985-2011!  This map suggests the GFS isn't entirely alone in forecasting a wet weekend, but we would like to see much better model agreement before buying off on this solution.

Now, if the rain does materialize, what impact will  this have on our temperatures?

7-day Spokane Forecast

Our current forecast is showing highs above average through the workweek with a cooldown by the weekend. Notice by the weekend we are forecasting highs right around 70 each day. That's not bad. Right around average actually. But we discounted the wet GFS solution. What if the GFS were to verify? How warm would it get then?

7-day temperature meteogram for Spokane

See the green line on the chart above? That is our official NWS forecast. However the blue lines (from the GFS) show the highs on Saturday could struggle to escape the mid-50s on Saturday and then slowly rebound into the lower to middle 60s for Sunday and near 70 on Monday.

Stay tuned to later forecasts this week to see how wet this weekend might really get. In the meantime, you might want to procure yourself one of these if you have camping plans this weekend.

How's tonight's trick-or-treat weather looking?

Yesterday we discussed what the odds of a wet Halloween would be and whether or not the holiday revelers would need to accessorise with umbrellas or additional rain gear. Well, now that the event is nearing our confidence in where rain will fall during the evening is growing. First lets look at the latest satellite picture. The feature of concern is the north-south band of clouds and moisture (while and green band) associated with a cold front (blue dotted line). This cold front is slowly working its way to the east and will likely it's trek through the evening.

3pm Water Vapor imagery with approximate cold front position in blue

So while the satellite was showing the cold front moving slowly to the east, what did that translate to in regards to precipitation? As you notice on the radar loop below, much of the preciptation has shifted east of the Cascades and Yakima (aside from some isolated showers over western Washington and Oregon) and was making a slow northeast path toward Spokane and northern Idaho. Based on this, it seems certain that rain will fall this evening over these locations, however the front is still expected to weaken during the evening. So where do we expect to see the best chances of rain during the evening between 6 pm and 9 pm?

Radar Mosaic from 130pm-300pm

To answer this question we will refer to the same ensemble forecast as yesterday (SREF model) as well as some hourly model guidance (HRRR model).

According to the SREF model, the best chances will occur over the extreme northeast portions of Washington and adjacent portions of north Idaho (areas shaded in purples and reds). This would impact trick-or-treaters in Sandpoint, Bonners Ferry, and possibly Colville. However notice the SREF isn't expecting much chance of measurable precipitation over Spokane or Coeur d'Alene(greens are 20% chance of less). This is because the front is still expected to stretch and weaken as it moves to the east this evening. The other area of concern for rain is over the extreme southeast corner of Washington, near the Blue Mountains and Pomeroy. Based on current radar trends, this seems overdone, but bears watching. Let's now check on the hourly HRRR guidance to see if it agrees with the SREF.

SREF measurable rain chances from 5pm-11pm

So below you will see the hourly HRRR model data for the period between 6pm-9pm. The images represent what the radar might be showing during those times. Greens and blues show where the model thinks there will be rain. Generally speaking it agrees quite well with the SREF, by keeping most of the rain to the north and east of Spokane. In fact, most of the rain generally remains fixed over northeast Washington and doesn't really hit north Idaho in force until after 8pm, and by then the front looks quite weak. Also notice that over southeast Washington the forecast looks dry until 9pm and then later into the night (not shown here).

6pm simulated radar from the SREF

7pm simulated radar from the SREF

8pm simulated radar from the SREF

9pm simulated radar from the SREF

Does this mean we can count on a dry evening over Spokane and Coeur d'Alene? We don't think so (the HRRR is showing a few tiny specks of green and blue around the area), The front has enough moisture and lift to produce some light rain in this area, but it won't likely amount to more than a few sprinkles and thus rain gear will be optional.

Rain or not, one thing is certain, this will be another mild Halloween. Much more so than what the region endured back in 2002. That evening, the holiday revelers had to endure temperatures in the teens. 2003 was not much warmer. However, since 2007, every Halloween evening has been fairly mild. Here's a look at trick-or-treat temperatures since 2000.

Halloween temperatures at the Spokane Airport since 2000

Severe thunderstorms tomorrow? It's possible.

After a seemingly endless barrage of weather systems, yet another is already beginning to show its stripes. Take a look at the satellite picture below. This is a water vapor image.Notice the green area covering the western portions of Oregon and Washington. That's associated with a plume of sub-tropical moisture which should deliver rain (and high mountain snow) to most of the Inland Northwest between tonight and Thursday morning.  Also notice the dark area over the eastern Pacific and the kink in the blue lines located along 130w. This is associated with a fairly vigorous shortwave trough. This trough will become a crucial player for our weather across the Inland Northwest by tomorrow afternoon.

10pm PDT Water Vapor image and 500 mb heights

So as mentioned before we expect widespread rains (and mountain snow) tonight into early Thursday. This will likely prime the atmosphere full of moisture and could set the stage for some active weather tomorrow afternoon across portions of the Inland Northwest. Lets look at some model data for details. First we will start with a course resolution model, the 90km GFS. Here's a look at what that models is showing as far as the shortwave trough moving through the region. The first image shows the previously displayed 500 mb heights combined with what we term Q-Vector convergence. This is a fancy term that essentially shows upper level lifting. In this case the purple shading shows where the lifting in the upper levels is strongest. The top image shows the trough extending from the central Washington Cascades to the SE corner of Washington around 11 am with the best upper level lifting focused over most of the region. The second image shows the same thing only at 5 pm. Notice by this time the trough is focused along a line from Sandpoint to Missoula, however the strongest lifting by that time has shifted into eastern Montana and SE British Columbia. That's all well and good but what does that translate to weatherwise?

11am PDT Thursday 500 mb heights and Q-vector convergence

5pm PDT Thursday 500 mb heights and Q-vector convergence

Lets take a look at one more 90km GFS image before moving on. This time we will look at CAPE values. CAPE or Convective Available Potential Energy is simply the amount of potential energy that can be released should we be able to lift an air parcel. You can read more about it here if you desire. Suffice it to say, the higher the CAPE values there more energy or explosiveness there is in the atmosphere. So what is explosiveness tomorrow afternoon? Its actually fairly impressive at least for this region and this time of year. Notice the axis of the highest CAPE values extend from the NE corner of Washington southeast toward the Clearwater Mountains southeast of Lewiston. Keep in mind this area of instability coincides with the passage of the shortwave trough and upper level lifting...a good thing for producing thunderstorms.

CAPE forecast for 5pm Thursday

The parameters shown above are what we meteorologist have looked at for many years, however we are now also getting more refined and specific data from finer resolution models and simulated radar data. So what is this newer model data showing? Its actually quite interesting. Here a look at simulated radar for 5 pm Thursday that was run 42 hours previously (at 11 pm Tue). Notice over NE Washington and the SE corner of Washington there are a pair of bright orange and yellow cells. This model was suggesting that there could be thunderstorms over this area. We see this a lot from the finer resolution models but often can't put a lot of faith in them unless there is some consistency.

Simulated 42hr Composite reflectivity (radar) from 11pm Tue Model run

 So has there been consistency? Lets see the next model run. This one was run 6 hours later and sure enough there are the two yellow and orange areas over NE Washington and a slightly larger one over SE Washington.

Simulated 36hr Composite reflectivity (radar) from 5am Wed  Model run

So that gives us some confidence. Now how about the latest model run? If you look at the image below you can see that the information isn't  quite as convincing as the previous two but it still showed two yellow and orange areas in the same general vicinity. Interesting. So is this enough information to convince us that we will see severe thunderstorms tomorrow afternoon. Probably not, but it does at least hint at a possibility of it. Our confidence is much higher that we will see some thunderstorms...a few of which will produce hail and possible some gusty winds. If the latest models were showing what we see depicted from northern Louisiana toward northern Illinois our confidence would be much higher!

Simulated 24hr Composite reflectivity (radar) from 5pm Wed  Model run