SPC AC 100559

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Mon Sep 10 2018

   Valid 101200Z - 111200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE MID-ATLANTIC AND SOUTHERN NEW ENGLAND...

   ...SUMMARY...
   A couple of severe storms may develop across a portion of the
   Mid-Atlantic states this afternoon and evening, continuing into
   southern New England tonight. The main threats will be a brief
   tornado or two and locally damaging winds.

   ...Synopsis...
   A closed upper low over the Lower Great Lakes will transition to an
   open wave and eject northeastward over Quebec today. A related weak
   surface low will also quickly develop northeastward along the
   US/Canadian border, reaching the Upper Saint Lawrence Valley by the
   end of the period. A trailing cold front will move eastward through
   NY/PA and the central/southern Appalachians, while a warm front
   advances northward across the Mid-Atlantic States through the
   afternoon, and into New England tonight. Farther west, a closed
   upper low off the British Columbia coast, and the leading extent of
   an associated positively tilted trough over the eastern Pacific,
   will move slowly southward toward the Pacific Northwest.

   ...Mid-Atlantic and Southern New England...
   Confidence remains low in the coverage of thunderstorms along and
   south of a warm front across the Mid-Atlantic this afternoon and
   southern New England tonight. More pronounced forcing for ascent
   associated with a northeastward-ejecting mid-level shortwave trough
   will likely remain to the north of the surface warm sector.
   Similarly, a strong low-level jet will also tend to become displaced
   northward across northern New England late this evening into
   tonight. But, some residual low-level warm air advection should
   continue over the Mid-Atlantic and southern New England in this time
   frame as the warm front lifts slowly northward. Forecast soundings
   from the NAM and RAP suggest that the increasingly moist low-level
   airmass coupled with a sufficiently veering wind profile in the 0-3
   km layer will support at least somewhat organized convection,
   although instability will remain weak owing to poor low/mid-level
   lapse rates. Isolated surface-based thunderstorms will be more
   probable along and south of the warm front through tonight, and some
   strong/gusty winds could occur. A brief tornado or two may also be
   possible, mainly with any thunderstorm that crosses the warm front
   and encounters relatively greater effective SRH.

   ...Northern Plains...
   A weak cold front moving eastward across the Dakotas early this
   morning will undergo frontolysis today as it stalls over western MN,
   central/eastern SD, and NE. Moderate instability will likely develop
   along this boundary by this afternoon as steep mid-level lapse rates
   and increasing low-level moisture will be present. Given modestly
   enhanced mid-level westerly winds, sufficient effective bulk shear
   should also exist that would support organized severe storms.
   However, only very weak convergence along the front and negligible
   large-scale ascent aloft suggests that convection will probably not
   initiate through peak heating. Some elevated thunderstorms may
   develop late tonight across far northern ND as a southerly low-level
   jet strengthens, but they should quickly move northeastward across
   the international border and into southern MB.

   ...Western/Central Carolinas and Vicinity...
   A subtle mid-level disturbance may aid in the development of
   scattered thunderstorms across the higher terrain of the western
   Carolinas ahead of a cold front this afternoon. Flow in both the low
   and mid levels is forecast to remain weak, which should generally
   limit updraft organization. Very isolated strong/gusty downdraft
   winds may occur across this region with ample diurnal heating
   steepening low-level lapse rates amidst a very moist low-level
   airmass. Regardless, the overall severe potential appears too
   marginal/disorganized at this time to include even low wind
   probabilities.

   ..Gleason.. 09/10/2018

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