After reviewing the ensemble forecast information, where do you think are
the best chances of a winter weather event meeting winter storm warning criteria
taking place? See the map of geographical
locations for definition of regions found in the choices below. For simplicity,
use the 4" of snow, 0.25" of ice in 24 hours, or a combination of
both as winter storm warning criteria for all regions. Check all that apply.
When finished, click "DONE" to see how well you did.
Discussion
The correct answer is the southern Mid-Atlantic States. Discussion of the ensemble forecast components supporting the answer is found below. We can examine the ensemble mean/spread and spagheeti diagram for large scale features which might suggest jet structures that would be supportive of significant winter weather (such as those found in Kocin and Uccelini, 1990), and the amount of certainty for the existence of these features. As we move from a 7-day to a 3-day forecast for the winter weather event, we note that:
250-hPa heights and winds
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Establishment of a upper-level polar vortex over northeastern Canada becomes more certain (spaghetti diagram and mean/spread). Uncertainty that remains by 84-hours prior to the event is associated with short waves in the polar jet stream rotating around the vortex. A trough in the eastern U.S. is forecast throughout, with the uncertainty decreasing with time. Ridges upstream and downstream of this trough are of relatively uncertain strength.
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In the ensemble 250-hPa wind field, we note convergence of potential solutions on the development of an upper-level jet streak over New England. The ensemble forecast of this jet streak steadily shifts north and east from its initial forecast position over the Mid-Atlantic states. The location of the jet at 84 hours is indicative of confluent flow over New England, which also leaves the Mid-Atlantic states in the left entrance region of the upper level jet streak. This area is associated with ageostrophic flow toward the warm side of the jet in the lower troposphere, which tends to keep cold air in place east of the Appalachians.
500-hPa heights
- Similar to 250-hPa, an upper level vortex is established, with an eastern U.S. trough and ridges both upstream and downstream. Confluent flow is established (seen in the height contour lines) over the northeastern U.S. and southeastern Canada.
- A short-wave trough at 500-hPa becomes more certain over the central and eastern U.S. within the general longwave trough. This feature is of high certainty (low spread) by 84-hours prior to the time of interest.
850-hPa heights and temperature
- Height trends in the mean and spread graphics indicate increased likelihood of troughiness (or perhaps even a closed circulation if the resolution of the ensembles were higher) associated with the 500-hPa short-wave mentioned above. Spaghetti diagrams of 850-hPa height show a similar trend, and increased agreement over time among the ensemble members.
- Temperatures at 850-hPa in the mean and spread trend colder, then warmer, and are near-critical over the Mid-Atlantic by 84 hours prior to the event. The spaghetti diagrams indicate that some ensemble members are above 5°C as far north as central VA.
Sea-level pressure
- The ensemble mean at 168 hours from the event shows a large area of high pressure over the center of the U.S. with low over the Canadian Maritime Provinces and northwesterly flow over the eastern U.S. By 132-hours, the ensemble mean is indicating an elongated trough off the U.S. east coast. This trough intensifies during successive forecasts so that by 84-hours from the event, a frontal wave is evident in the ensemble mean sea level pressure.
- Spaghetti diagrams of 1008 and 1032-hPa sea level pressure indicate no members with a closed isobar near the U.S. east coast at 168-hours from the event. Two members of 11 have a closed 1008-hPa isobar at 156 hours, six at 132 hours, and by 84-hours prior to the event, all ensemble members have a closed 1008-hPa contour, with the uncertainty being in the position of the low center (anywhere from the central AL/GA border to east of Norfolk VA), rather than in its existence.
24-hour Precipitation amounts and diagnosed precipitation type
- As might be expected from the above discussions, the ensemble prediction system did not indicate greater than 30% potential for more than 0.25" of precipitation in the Mid-Atlantic states until 132 hours prior to the event. No more than 30% of the ensemble members indicated more than 0.5" of precipitation in 24 hours until 108 hours from the event over any part of the Mid-Atlantic. At most only two ensemble members indicated 1" or more of precipitation in 24 hours until 84 hours prior to the event.
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The precipitation type ensemble forecasts indicate little chance of snow (about 10-20% or 1-2 ensemble members), and no chance of sleet or freezing rain at 168 hours prior to the event. By 84-hours prior, there is at least a 50% chance of snow from the northern third of VA to northern PA and eastward to the New York City area. The northern portion of this area is not forecast to get more than 0.25" of liquid equivalent. Of even more concern is the sleet and freezing rain diagnostics, which indicate a significant risk of sleet and freezing rain (40-50% for ZR, 30-40% for IP) in southern VA and NC, where 1" or more of precipitation is indicated by all of the ensemble members.
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