20:08 Tue Apr 17, 2018
Spring Around the Corner?
With how cool the weather has been across much of the country,
many are looking ahead to see when more spring like conditions will begin. First, why has much of the eastern United States
been seeing such cool weather when we had such warm conditions in February and
early March? Well in the overall synoptic flow (Large scale weather patterns
larger than 1000km across) there has had consistent blocking high pressure over
Greenland, which creates a deep trough (Dip in the jet stream) resulting in
cooler than average temperatures. Figure 1 below is a plot of the 500 millibar
height anomalies for the northern hemisphere over the past month. The warm
colors over Greenland and the colder colors over Canada are a good indicator of
the persistent blocking that we have been seeing over the last month.
Figure 1: 500 mb height anomalies in the northern hemisphere over the last month.
Higher than average snowfall in March and early April has recovered the snowpack lost in the February melt and then some. In Figure 2 and 3 below, snow covers much of the interior North East as well as the Great Lakes region to the Hudson Bay. With northwest flow due to the persistent blocking pattern, air masses are originating from over the deep snow pack. The snow helps insulate cold air as it moves south by both reflecting incoming solar radiation with any heating from the sun going into melting the snow, keeping temperatures near freezing. If there was bare ground, the air mass would heat up during the day as the sun warms the ground and transfers that heat to the atmosphere above it. With how strong the sun is at this time of year, any snow free ground would quickly warm a polar air mass and take away its chill.
Figure 2: Current snow depth across the United States
Figure 3: Current snow depth across the North East
The good news about this map is that there is not much snow to the south and southwest of New England. Once the weather pattern changes and we get a few days of southerly or southwesterly flow, the snow in the North East will quickly melt, exposing the bare ground to the sun to really start our spring season. We would need a weather pattern with southerly flow to bring in the warm air with sunny skies to last long enough to melt a vast majority of the snow in the Great Lakes region and southern Canada. This will give us a good buffer from the colder air masses as they travel out of the Polar Regions. Losing all that snow won’t necessarily mean that we will be free from the cold weather but the amount of time that it will stick around will be greatly reduced.
Looking ahead in the short term, we will have another colder storm coming in keeping below normal temperatures into the weekend, but beyond that is much more promising for warmer weather. In Figure 4, I have a plot of the 500 mb height anomalies expected early next week. It is a long way out so I am using an ensemble of model to get more of a mean out of a bunch of different model runs with different parameterization to help reduce error in the long range. Looking at anomalies also gives a better idea on if we are going to see a better chance of warmer or colder weather rather than looking at specifics like precipitation or expected high temperatures.
Figure 4: Forecasted 500 mb height anomalies for early next week
With those darker oranges over New England, there is good agreement that a warm up is on its way. We will have to see how much snow and ice this warm up will melt! This late into spring, it is possible that after this weekend southern and coastal regions of New England will have seen the last of below freezing temperatures until fall! Up here on the Rockpile, we are prepared to see winter for another few months of winter! We can see snowfall up here in any month of the year!
Adam Gill, Weather Observer/IT Specialist