On Tuesday April 7th we were able to get in our weather discussion just in time before the campus-wide power outage hit (not weather-related). We looked at the current surface map, which showed noontime temperatures in the upper 60s over the DC metro region, with dew points in the 50s under mostly cloudy skies. A back-door cold front, however, was bearing down on us from the northeast. At time of discussion, it appeared to have just crossed through northern New Jersey, where winds had shifted to the northeast direction and temperatures were only in the upper 40s. This front was forecast to continue sliding southwestward and cross our area overnight. Wednesday and Thursday were forecast to be cloudy, cool, with a northeast wind directly off the chilly Atlantic Ocean…so under those conditions, expect highs barely topping the 50 degrees mark, if that.
So what’s the deal with these “back-door cold fronts,” anyway? (Their name of course comes from the fact that they approach from the northeast, and even sometimes due east direction, in contrast to the usual west-to-east moving midlatitude cold fronts.) Well, they make frequent visits to the northeast U. S. in the early spring, particularly when high pressure settles over the New England area and the westerly winds become weaker, as they tend to do this time of year as the polar jet begins its northward trek. New England deals with them most frequently, but they often push as far south as Virginia and sometimes deep into the Carolinas. The still very cold ocean water temperatures off the northeast coast also help to reinforce the surface high pressure, giving an additional push to the cold dense surface layer that would love to spread southwestward and spoil spring weather in the DMV. Even though the global models now seem to be able to get a good handle on these fronts’ movement several days in advance, forecasting the exact timing and southwestwardmost extent remains somewhat tricky becuase their leading edge is oven a very shallow layer of cold air that undercuts the warmer airmass currently in place.
Otherwise, this looks to be an active severe weather week across the central part of the country. Surface mesoanalysis shows a warm airmass over much of the Midwest/Great Plains/Dixie region with fairly juicy dewpoints for this time of year (> 60 degrees F) sitting south of a southern Missouri/central Kentucky/east Tennessee/North Carolina line. Not coincidentally, several mesoscale convective systems can be seen riding this dewpoint gradient, with the most intense activity forming a bow echo near the St. Louis area and with several severe thunderstorm warnings associated with it. We discussed how convection often likes to become more aligned with low-level moisture gradients, rather than the polar front itself, even though the temperature gradients are often much more pronounced across the polar front. For example, despite its 20-degree temperature change across a short distance, our backdoor cold front had no convection associated with it at all. A lot of this has to do with the fact that convection tends to stay near the deeper moisture source, and then new “fronts” end up forming after a series of convective complexes ride along the dewpoint boundary and reinforce the north-south temperature gradients across it with their cold, dry outflow to directed primarily to the north.
We also looked at the severe threat for wednesday, as a digging trough over the Rocky Mountain Region is forecast to send a shortwave trough/vort max over the Kansas/Oklahoma area late in the day. There will be plenty of instability, with CAPE forecast to exceed 3000 J/K. This strong instability is thanks to 1) plentiful low-level moistude (mid 60s dewpoints) and 2) the presence of an Elevated Mixed Layer, or EML, over much of the midwest region. EMLs feature a strong inversion centered around 700-800 mb with very steep lapse rates (> 7 C/km) above. The steep midlevel lapse rates owe themselves to the dry Rocky Mountain origin of these airmasses. They are often the real culprit in a lot of high-end severe weather events that plague the Great Plains/Midwest Region, but they do on occasion overspread even the mid-Atlantic and Northeast (part of why the Dulles Airport sounding just prior to the June 29, 2012 Super Derecho showed over 5000 J/kg CAPE was that the strong central U.S. upper-level high had advected an EML over and east of the Appalachians, but of course the 100+ degree surface temperature/75+ degree dewpoints also factored in). Why are EMLs associated with severe weather? First, the low-level inversion, or “cap”, acts as a lid on cumulus development, allowing the boundary layer to get plenty of insolation-it effectively acts like a lid on a boiling pot of water. Then, when and if a lifting mechanism can cause convection to break through the cap, the updrafts have a huge amount of CAPE to tap into, thanks to the rapidly cooling environmental temperatures above the cap. When forecasting severe weather for environments capped by an EML, the tricky question becomes “if and when will the cap break and storms erupt?”. Such a situation looks to be the case for Oklahoma and Kansas tomorrow evening, but it appears likely that the vort max approaching from the Great Basin upper level trough should supply enough of a lifting mechanism to cause storm initiation eventually – although areal coverage of storms there is still a big question. For this reason the Storm Prediction Center (SPC) has placed much of that area under a 15% Slight Risk but with a hatched area showing a 15% probability of “high-end” severe weather (i.e. strong tornadoes, hurricane force winds, very large hail). They often do this when they see the potential for some very nasty severe weather but they don’t want to go full-on with a Day 2 Moderate Risk, due to uncertainties about whether the storms will actually initiate and how widespread they will actually be.
Finally, it looks like we might get a shot at seeing some thunderstorms friday afternoon as a warm front lifts through, especially if the sun is able to get out for a time in the afternoon. The storms would most likely arrive in the late afternoon or evening ahead of a cold front, and some scattered strong to severe storms are a possibility if enough instability can develop ahead of the convection.
A classic Great Plains EML sounding is shown. Note the very sharp inversion between 750-800 mb, and the very steep lapse rates above.
