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.

Will the Inland Northwest see snow on Sunday night and Monday?

The Spring thus far has been fairly mild, and as a result we haven't had to worry about the threat of snow. The temperatures for March were some of the warmest if not the warmest on record for many locations.

March temperatures

But all weather patterns come to an end at some point and this will be no exception. So here is what the upper-level weather pattern for most of the month of March looked like. It showed a rather persistent ridge focused over the West Coast with the storm track nudged well north of our area

Mean 500 mb pattern for March 1-March 29th

However since the end of the March and continuing into April, the upper-level ridge has shifted to our east, allowing the upper-level trough to nudge into the eastern Pacific and the storm track to push through the Pacific Northwest.

Mean 500 mb pattern March 30th-April 1st

This has resulted in more typical springtime weather for the region including significantly cooler temperatures, and numerous afternoon/evening showers and thunderstorms accompanied by an Inland Northwest springtime staple: graupel. But despite the cooler weather, most of the valley locations, have yet to deal with snow. That might change though with the arrival of the next significant weather system. The model solutions are rather convincing that this system will arrive sometime between late Sunday night and Monday as the deep offshore trough heads inland.

500 mb pattern for Monday morning

Referring to  the map above, it suggests most of the upper-level energy will remain poised off the southern Oregon/Northern California coast, however look closely over extreme northeast Washington/north Idaho and notice the small yellow circles. These indicate the larger trough will also contain several small shortwave troughs.  If these troughs are accompanied by enough moisture and instability they can trigger small bands of moderate to intense precipitation, or what we
meteorologists term meso-banded precipitation or mesobands. Even more intriguing is these mesobands can often allow snow to fall (and possibly accumulate) at much lower snow levels than expected.

The odds are quite good that the region will see mesobands of precipitation form, the big question is where and what will the impacts be?

To answer the first question of where will these bands form, we first need to look at the positioning and track of any surface low tracks. Here is a look at four weather models we typically utilize and where they place the low (and resultant precipitation) as of 5am Monday morning.

4 model solutions with surface low position and 6-hour precipitation valid 5am Monday

As you can see above, all four models have a surface low located over the eastern third of Washington or north Idaho, however, the exact positioning and strength of the low is quite variable. The solution in the upper left corner is by far the most impressive solution. It has the strongest surface low (1000 mbs) and the heaviest amount of precipitation. The purple colors in this image represent precipitation amounts between 0.50-0.75" in a six-hour period. The other model solutions show a weaker low (1005-1006 mbs) and much lighter precipitation amounts.

When there are wide disparities between our core models we like to defer to ensemble modeling. This is where we take an initial model run and add small perturbations to the mix. The perturbations begin small but with time tend to grow. When the models cluster the positioning of a low, this boosts our confidence in the forecast, whereas if lows are strewn haphazardly across the region, our confidence is quite low. So here's a look at the SREF positioning of the surface lows for Monday morning.

SREF Surface low positions for 5am Monday

Although there are plentiful lows (L's) found across the Inland Northwest there is a wide scattering of their positions. The fact that there are plentiful lows is good from a standpoint of there is fair confidence of an event occurring. However, the wide scattering of the L's lowers our locational confidence significantly.  We can also look at the mean of all the low positions and the mean of all the precipitation data to come up with a preliminary snow forecast. In this case, the SREF is showing this as the mean snowfall for the 12hrs ending at 11 am Monday.

Mean 12hr snowfall ending 11am Monday

While these values themselves are not impressive (1-2" for the areas shaded in green) the fact that this model is showing such widespread snow is noteworthy. Another interesting thing we can look at from the SREF model is a plume diagram which shows all the suite of model solutions on one chart. So given the map above, it would seem locations north and east of Spokane as well as near the Cascade Crest would see the best chances of snow. So to hone our forecast a little more we will look at some plume diagrams to determine potential snow amounts. The first we will refer to is for Sandpoint.

Sandpoint snow plume diagram

Each line on this plot refers to one particular member of the SREF ensemble. In this case, most of the solutions are showing some snow on the right side of the diagram. The mean of the runs is denoted by the black line which is indicating a mean snowfall of 2". More impressive (but not probable) is the pink line which shows snowfall totals exceeding 8".  How about the plume diagram for Spokane?

Spokane snow plume  diagram

Even the Spokane plume diagram is showing some snow. Nothing compared to Sandpoint, but some snow nonetheless. The mean snowfall is right around an inch, however, there is one run which shows  totals nearing 5"!

Now despite these snow forecasts, the other factor to consider is how easily will this snow accumulate on the ground? The temperature forecast for late Sunday night and early Monday morning is for readings in the lower to middle 30s. Certainly cold enough for snow, but perhaps not enough for significant accumulations, especially in the Spokane area. After sunrise, temperatures will slowly climb into the upper 30s to middle 40s which suggest snow accumulations are even less plausible except perhaps on grassy surfaces. 

So in summary we are fairly confident a deep but compact surface and upper-level low-pressure system will track through the Inland Northwest producing locally moderate to heavy precipitation some of which will fall as snow. Where it will go and what time of day it hits will be critical for determining what it's impacts will be. Winter driving conditions are certainly possible for the Monday morning commute, especially north and east of Spokane. In the meantime, stay tuned to our latest National Weather Service Forecasts and don't put away your winter clothes quite yet. 

Unusually wet February and March

Greetings folks, sorry it's been a while since we last posted to this blog, but we have been focused on implementing a new computer system for our office used to compose our complete suite of products and weather grids. Now that we have gotten our feet wet, it's time to discuss the unusually wet February and March the Inland Northwest endured.

So the weather pattern that took a dramatic shift in February unexpectedly continued into March. What was the cause? Recall that a highly anomalous ridge was fixed over the extreme eastern Pacific through the first half of the winter. Here's what it looked like on a 500 mb chart (approximately 17-18k ft). Notice the strong buckling in the flow just off the west coast. This is rather unusual and resulted in almost a record breaking dry spell for the first half of a winter.

500 mb mean map for 10/1/13-1/15/14

Well as the calendars changed to February, the unrelenting ridge gave way and opened the doors to countless storms which surged through the Pacific Northwest for the following 2-month period.
The mean 500 mb charts for February and March exhibited this ridge flattening.

500 mb mean map for 2/1/14-3/29/14

Notice although the ridge is still present (albeit flatter), its actually a much more favorable setup for precipitation since the amplitude of the ridge allows sub-tropical moisture to be wrapped into passing weather systems. Here's is what the anomaly of the atmospheric moisture (called precipitable water) looked liked for the period. Notice the well defined connection of moisture extending from southwest of the Hawaiian Islands northwest to the coast from northwest California to southern Washington (red, orange and green shading).

Precipitable Water Anomaly 2/1/14-3/29/14

So how wet was the two month period? Very was the answer. The map below shows the departure from normal of precipitation for February and March. Every location saw wetter than normal conditions, with the Cascades and Idaho Panhandle mountains leading the pack with well over 10" more than the normal amount of precipitation.

Rainfall departure from normal February & March 2014

From a percentage standpoint vs. normal it was also an impressively wet period. Note the dark blues and purples over the Cascades and north Idaho mountains, indicating where precipitation amounts were 2 to 4 times the normal for that period.

Rainfall % of normal February & March 2014

The unusually wet period has also brought the snow water equivalent (amount of water in the mountain snow pack) to normal or just above normal over the entire region. Note that although conditions were moist over Washington, Idaho, and western Montana, things were still quite dry across Oregon and California (not shown).

Not only did a considerable amount of precipitation fall, but it also did so on an unusually high number of days. Here's a look at the numbers compared to the normals.

Days of measurable rainfall for various locations across the Inland Northwest Feb-Mar 2014

Over locations where the temperature was cold enough, most of this equated to snow. In Holden Village, WA the snowfall for the two-month period was 180.7". That shattered the old record of 166.7" (set in 1999). Not quite as impressive was the 14.0" which fell in Lewiston, ID. This was the most since Feb-Mar in 1985.

So will this unusually wet weather continue? Showers are still fairly common this time of year. In Spokane we typically see some precipitation about 50% of the time during the first half of April. That number drops to around 40% of the days by the end of the month.  In Wenatchee and Moses Lake, the percentage of wet April days is a mere 25%!

The 8-14 day outlook for precipitation shows near normal conditions for most of eastern Washington and north Idaho.

8-14 day precipitation outlook.