Forecaster Discussion

Current Discussion:

SAT NIGHT UPDATE

FRIDAY RECAP

The event on Friday verified well against SPC's 6Z outlook. A forward-propagating MCS was able to maintain itself down to the Gulf coast. Scattered wind damage/gusts were observed along the MCS path consistent with an Enhanced Slight risk. Outside of two wind reports in southern LA, my thoughts on southwestward backbuilding did not materialize. There were very isolated hail reports and no tornado reports, consistent with outflow-dominant convection.

SATURDAY RECAP

There were multiple high-precipitation supercells that developed mainly over eastern NE and moved into E KS during the afternoon. These had access to >4000 MLCAPE and ~60kt of 0-6km shear, and unsurprisingly one of these storms produced at least one (likely multiple) tornadoes and a measured 5" diameter hailstone. These storms were evidently triggered by subtle low-level/boundary layer features. This event was missed entirely by both the SPC and myself during the issuance of the initial discussion, and highlights the difficulty in forecasting subtle forcing mechanisms that can really make a splash when dealing with CAPE in the thousands beneath strong upper level winds at the periphery of an upper ridge. Other severe storms were observed over parts of MT/WY/SD, possibly associated with the wave identified during the initial discussion.

SUNDAY FORECAST

PLAINS-- Overall thinking has not changed much since the initial discussion. A more pronounced wave should kick off multiple clusters of thunderstorms from CO/KS up through Montana. Boundary layer moisture quality won't be that great this far west, however very steep lapse rates from the sfc through 500mb should allow MLCAPE of 1000-2000 J/kg to build in this region (higher with southeastward extent). 0-6km shear will range from 50kt to over 80kt (higher with northward extent) and curved hodographs will be favorable for supercells. Steep mid-level lapse rates and strong mid-upper level shear will be favorable for large hail with several 2"+ reports likely. Deep boundary layer mixing (greater with SE extent) will be favorable for severe downbursts as well. High LCLs and weaker 0-1km shear will limit the tornado threat somewhat, but at least a couple reports are expected. Storms will likely congeal into one or more clusters and likely sustain through the end of the period. Shear and high elevated CAPE will likely maintain a severe threat as well.

S ILLINOIS REGION-- A wave dropping out of the NW will interact with MLCAPE around 2000 J/kg and deep-layer shear of 20-30kt to develop/intensify storms in this region. Storm mode should be multicellular or quasi-linear due to the relatively weaker shear, though somewhat curved low-level hodographs may allow for embedded supercells. Hail, wind, and a tornado or two are possible with these storms.

VIRGINIA REGION-- Scattered thunderstorms are expected to develop during the afternoon in an environment with 1000-2000 J/kg of MLCAPE and 30-40kt of 0-6km shear. Straight hodographs may allow for a few splitting supercells (modeled by the 00Z NAMnest). The primary threat here should be wind as weak mid-level lapse rates may curtail the hail threat.

MONDAY FORECAST

MIDWEST INTO OHIO VALLEY-- Main change this update is to highlight a severe potential in this region. The 00Z CAMs are in good agreement in some sort of severe MCS evolving or developing somewhere near the upper Mississippi Valley region in the morning and propagating E or SE for some distance depending on the model. A warm front will be moving north through this region beneath strong westerlies and very steep midlevel lapse rates. Dew points south of this warm front will be well into the 70s, which combined with the steep lapse rates will allow CAPE values possibly well over 3000 J/kg. Should a MCS develop and latch onto the more unstable airmass, a progressive derecho is possible in this region with a swath of significant wind damage.

DAKOTAS-- The general idea from the previous discussion still stands. The upper trough will start to push into this region into an unstable airmass and cause favorable conditions for severe thunderstorms. Currently the CAMs are initiating storms over MT/WY in the afternoon and lifting them northeast through the evening hours. A second area of storms may develop over eastern ND into MN. Given the instability and shear, all types of severe weather are possible which could include several tornadoes if the greater low-level shear verifies and storms mature before the boundary layer stabilizes.

Forecast discussion by Chris Kwinta

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