The pattern is changing to a wetter one as the dip in the jet stream in the East shifts west and introduces moist southwesterly flow across the area. That change will also introduce moderating temperatures this weekend as a low scoots to our north supplying southwesterly low level winds.
Despite the southwesterly flow aloft that is expected to dominate the next week or so, this still is not a really warm pattern. Rather, it’s an interlude of normal to slightly above normal temperatures that will probably transition back to colder than normal temperatures towards February 6 or 7.
The period between February 2 and 6 looks like it will be a stormy one, but more rainy than snowy. However, this is a pattern with warm air possibly alternating with short cold shots so it’s not a pattern where you can completely rule out the possibility of a wintry mix or snow.
Our daytime highs are likely to be back in the 40s by this weekend and I wouldn’t be surprised if we nudged above 50 sometime. However, beyond this weekend the future gets hazy as our location is very close to where warmer than normal temperatures will be battling colder than normal ones for supremacy.
The uncertainty about which side of the cold versus warm fence we’ll be sitting on at any given time is reflected in the large spread in the temperature forecasts being shown in the simulations of GFS model ensemble below.
The individual ensemble members (simulations) are indicating significant uncertainty about the temperature forecasts beyond the weekend.
(Remember that ensemble members are essentially the GFS model run at a lower resolution than the operational model with each member having slightly different initial conditions. Meteorologists tweak the initial conditions because we know that there is no way to get perfect observations at every point in the atmosphere around the globe. These slight tweaks allow us to assess how sensitive the pattern is to slight differences in the initial conditions. In this case, the ensembles are indicating there is a lot of sensitivity.)
A few members are predicting the maximum temperature on February 5 will exceed 60 F while the average of all forecasts (the black line) is stuck in the 30s. That huge difference suggests not wedding yourself firmly in any longer range forecast over the next week or so. Some of the ensemble forecast differences are due to different timing of fronts between the various ensemble members but the differences also reflect that some models take a low around February 4 or 5 south of us keeping us in the cold air, while others track the low north of us, placing us in the warm sector.
This same uncertainty is reflected in the ensemble forecasts of precipitation and precipitation type during the next 7 days. When the lines are climbing the fastest, the precipitation is falling the heaviest. When the lines parallel the bottom line or x-axis, no precipitation is falling. The big take away from the ensembles is that the pattern looks like a wet one during the February 2-6 period but with little consistency about when individual storms might start and end. Neither the pattern nor the ensembles are very bullish about the prospects of snow during the next week though some members are calling for brief periods of snow and/or freezing rain.
Beyond February 7, the pattern may revert to a cold one again and, while perhaps not as stormy as the pattern preceding it, one that is above normal for snow prospects.
Pattern discussion for February 2-7
The 5-day mean 500 mb height (atmospheric pattern) and anomaly (difference from average) map (below) provides clues about why there is so much uncertainty in temperatures going forward and also go long way towards explaining why the pattern should be a wet one.
The steering currents and upper level winds roughly parallel the black height lines on the map above. The above normal heights (red area) over the East indicate a building subtropical ridge which normally would result in above normal temperatures across our area as low pressure systems exiting the southwest would track north and west of us putting us on the warm side of the front. The models are indeed forecasting that a couple of lows will track to our north this week. Complicating the situation is the massive ridge over Alaska and the above normal heights extending across northern Canada into Greenland. That ridging forces northerly flow across most of western Canada steering cold air masses into the northern Plains.
The trough in the West (blue area) on the map above and strong ridge over Canada combine to yank colder than normal air into most of the county especially the Northern Plains (see below). The Southeast ridge does keep the Southeast warmer than normal and we end up with near normal 850 temperatures during the period. Each time a low tracks to our north and gets east of us, another shot of below normal temperatures press through the region. Weak waves on these trailing front offer opportunities for overrunning precipitation events. The ridging over Greenland and the negative anomaly just to its south also suggests that cold high pressure systems might also track to our north giving us chances for frozen precipitation before changing to rain.
The analogs based on past patterns similar to the European model simulations were a mixed bag but most did not offer much in the way of snow. The one exception was early February 1996 which features two weak waves that produced overrunning with enough cold air to support snow. By contrast, half of the analogs yielded a day with temperatures in the 50s to low 60s, again emphasizing how tricky trying to predict the temperature departures from normal during the period.
Pattern discussion February 7-12
Both the European ensemble mean and the GFS suggest that 500 mb heights will be well below normal south of Hudson Bay (darker blue shading) into the Great Lakes region indicating that blocking remains in place across the poles forcing the polar vortex south of its normal position. Both still suggest plenty of cold air will be entering the country.
The black lines extending from the poles into the Great Lakes region on the European model mean below suggest that the steering flow will still bring cold air masses into the Plains and Midwest and eventually that cold air will spread into our region.
The southward extent of the below normal heights on the European model suggests that a large portion of the country could experience below normal heights if the model is correct. That’s always a big if even when just looking at the larger scale features. Some flow is also breaking through the West Coast ridge, so a few systems of Pacific origin could run along frontal boundaries wherever they sets up. The pattern does not look as wet as the pattern that will be in place through February 7 but still looks like it could support above normal chances for snow. However, the analogs based on the 5-day period ending Feb 12 are surprisingly devoid of significant snowstorms.
The mean 850 mb temperature shown above illustrates how extensive the cold could be. The GFS ensemble mean forecast for the same period would also keep most of the country below normal except the Southeast and would keep us closer to the frontal boundary between the colder than normal temperatures and warmer than normal ones. Its variation of the pattern would probably increase our chances for getting overrunning precipitation suggesting the pattern would remain wetter than normal. I’m opting for a compromise between the two camps keeping our average temperatures a tad below normal while opting for above normal precipitation. The combination should offer winter weather chances in one form of another.
Overall the pattern during the next 2 weeks looks to be a wetter than normal one. Temperatures are a tough call as there is likely to be a periods when lows go to our north leading to warm days and then cold days when they exit the East Coast. The period after February 6 or 7 looks to be more favorable for snow than the period prior to Feb 6 and looks like it will also be colder.