By Dan Bikos, Sheldon Kusselson and Jorel Torres
On 30 November 2020, a trough affected the Mid-Atlantic area and was accountable for the next extreme climate experiences:
On this weblog put up, we’ll look at features of the synoptic scale setup for this occasion.
The next GOES-16 low-level water vapor (7.3 micron) animation:
depicts an extra-tropical cyclone transferring northeastward. Thunderstorms developed in northeast Virginia round 1800 UTC and moved northeast throughout Maryland, Delaware, Pennsylvania and New Jersey. These thunderstorms seem to have developed on the nostril of a dry slot obvious within the imagery as a area of hotter brightness temperatures. Click on on the picture under to see the thunderstorms of curiosity (yellow circle) and approximate boundary of the dry slot over the area of curiosity (inexperienced line):
What function did the dry slot have on the thunderstorms?
One of many extra helpful observational merchandise to evaluate atmospheric water vapor in 4 dimensions is the Advected Layered Precipitable Water (ALPW) product which makes use of microwave devices on a number of polar orbiting satellites. Water vapor is retrieved with out the usage of NWP output within the retrieval, short-term NWP output wind data is used solely to advect the retrieved moisture fields. A loop of ALPW appears like this:
Higher-left panel: Floor to 850 mb Precipitable Water
Higher-right panel: 850 to 700 mb Precipitable Water
Decrease-left panel: 700 to 500 mb Precipitable Water
Decrease-right panel: 500 to 300 mb Precipitable Water
The loop goes again to 1800 UTC the day gone by in order that we might view how the moisture fields are altering in time. At low-levels, areas of upper moisture point out the nice and cozy sector with moisture advection northward forward of the growing cyclone. As we get increased into the environment we see indications of the higher low in northern Texas / Oklahoma with drier air on its southern / southwest flank wrapping cyclonically round it on the japanese flank:
Subsidence throughout the dry slot would account for dry air at mid- to higher ranges, however are there different sources of dry air?
Subsequent, we’ll contemplate this 4 panel animation:
Higher-left: GFS 700-500 mb lapse fee
Higher-right: ALPW product within the 700 to 500 mb layer
Decrease-left: GOES-16 7.3 micron imagery
Decrease-right: ALPW product within the 850 to 700 mb layer
Early within the loop, through the daytime hours of November 29 we see a slightly apparent area of steeper mid-level lapse charges in Arizona and northwest Mexico. The night sounding from Tucson, AZ:
Reveals a deep well-mixed layer with very steep (practically dry-adiabatic) mid-level lapse charges. Within the GOES 7.3 micron imagery we see very heat brightness temperatures because the weighting perform peaks round 600-700 mb, throughout the properly blended heat / dry air mass. The ALPW product within the 700-500 mb layer exhibits this as a really dry air mass as properly. The animation signifies that this air mass advected eastward in direction of Texas, so we might suspect some portion of the dry air noticed additional east might have come from the Elevated Combined Layer (EML) throughout Arizona and northern Mexico. Subsequent we’ll take a look at soundings to evaluate this.
The 00Z soundings from Del Rio, TX:
and Corpus Christi, TX:
depict dry air at mid-levels, in addition to very steep mid-level lapse charges attribute of what you could anticipate with an EML as we noticed within the Tucson sounding.
The animation seems to indicate the dry air mass transferring from Arizona into Texas and Louisiana combining with the growing trough the place you’d anticipate to see a subsidence sign (mid-level drying). The truth is a sounding north of the EML plume as indicated by the steeper mid-level lapse charges within the GFS at Fort Price, TX:
exhibits the dry air mass at mid-levels, however not the steeper mid-level lapse charges like we observe additional south in Texas, very similar to you’d affiliate with being in a subsidence area of a growing trough. The dry air mass we observe on satellite tv for pc photographs seems to be a mix of each the subsidence area of a growing extra-tropical cyclone and an EML with origins over Arizona / northern Mexico. The EML is of specific notice because the steeper mid-level lapse charges might result in a extra favorable surroundings for extreme thunderstorms. Click on on this picture to check the above 00Z sounding websites with the imagery on the corresponding time:
Subsequent, let’s assess November 30 12Z soundings, we’ll be referring to the next sounding websites (corresponding imagery from 12Z is proven):
Was the EML evident within the Nov. 30 12Z soundings?
Sure, in Atlanta, GA:
and likewise in Tallahassee, FL:
Notice that the area of hotter brightness temperatures within the GOES 7.3 micron band and drier air mass within the ALPW 700-500 mb layer will not be fairly to central North Carolina at 12Z:
The mid-level dry air mass had not but arrived at sounding time, nevertheless with out an 18Z RAOB is there one other technique to assess the air mass at Greensboro, NC (GSO)?
Sure, NUCAPS supplies soundings (besides no wind knowledge) round 18Z, right here is the NUCAPS sounding at the moment a bit of southeast of Greensboro, NC throughout the dry mid-level air mass:
We are able to see that the mid-level dry air mass has moved by means of Greensboro, and the mid-level lapse charges are fairly steep (7.5 levels C per km), that is affirmation of the mid-level lapse charges brought on by a mix of the EML and being within the subsidence area of the extra-tropical cyclone.
Examine the 12Z Greensboro sounding with the 18Z NUCAPS sounding close to Greensboro
Notice that low-level options resembling sharp capping inversions might not be adequately resolved with NUCAPS soundings, nevertheless that’s not a difficulty right here since we’re assessing mid-level drying and lapse charges.
Shifting on to the time of the extreme thunderstorms, we analyze the GOES seen (0.64 micron) imagery:
Cloud protection was fairly in depth through the morning hours over the area that skilled extreme climate which probably restricted the general extreme risk, nevertheless there was a slender area of clearing that developed that allowed for ample insolation and thus destabilization that contributed to the extreme thunderstorms.