Monday, May 18, 2015

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.

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