SPC AC 251952
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0252 PM CDT Fri May 25 2018
Valid 252000Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE UPPER MISSISSIPPI VALLEY AND SOUTHERN PLAINS...
Severe thunderstorms are possible this afternoon into the evening
over parts of the upper Mississippi Valley and the southern Plains.
A few strong to severe storms are also possible over the interior
Northwest and northern Maine.
A couple changes have been made to the outlook for this issuance.
The first change is to expand the slight risk area in the upper
Mississippi Valley westward across most of western Wisconsin where
Severe Thunderstorm Watch 120 is located. The second change is to
add a 2 percent tornado and significant hail contour in parts of
western Oklahoma/eastern Texas Panhandle southward into northwest
Texas where strong instability is analyzed by the RAP and regional
WSR-88D VWPs show moderate deep layer shear.
.PREV DISCUSSION... /ISSUED 1121 AM CDT Fri May 25 2018/
A long-lived MCS has spread southward across OK into north TX, with
the associated outflow boundary now extending from the Childress, TX
area eastward along the Red River. 12z CAM solutions have not
accurately portrayed the extent of the cold pool, and are likely too
bullish on recovery of the air mass over western OK later today.
Nevertheless, a combination of northwest flow aloft and steep mid
level lapse rates will pose a risk of large hail and damaging winds
with any storms that can redevelop in this region this afternoon and
evening. Have shifted the SLGT risk area slightly farther west to
capture this region.
The leading edge of the MCS will move into north TX today, with a
MRGL risk of damaging winds or hail through the afternoon.
Overnight thunderstorms are slowly weakening across WI, but have
left multiple weak surface boundaries that may help to focus
thunderstorm development later today. Model guidance is somewhat
diverse in timing and location of initiation, but it appears that
this region will have some risk of damaging winds and hail in the
strongest cores. 12z guidance provides considerably lower
confidence in the severe threat farther southwest into IA, so have
trimmed the SLGT risk in this area.
Stabilizing effects of the overnight MCS over OK have lessened the
risk of isolated strong/severe storms in KS and western MO, so have
removed the MRGL here.
Scattered thunderstorms are expected this afternoon along a surface
boundary extending from Quebec across northern ME. Forecast
soundings suggest the potential for a few fast-moving cells capable
of locally damaging winds and hail.
Strong heating is occurring across this region today, with dewpoints
in the 50s yielding afternoon MLCAPE value up to 1000 J/kg.
Thunderstorms are expected to develop over northwest NV this
afternoon and spread northward into southeast OR. Other more
isolated storms will form along an axis extending into western ID.
Strengthening flow aloft and sufficient CAPE will promote the risk
of a few strong cells capable of gusty winds and hail. Parts of
this area may require an upgrade to SLGT risk this afternoon if
mesoscale trends warrant.
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