U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 220531 
Storm Prediction Center ac 220530 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1230 am CDT Wed Aug 22 2018 

Valid 221200z - 231200z 

..there is a marginal risk of severe thunderstorms today across 
parts of southern New England...the southern middle Atlantic 
coast...the Central High plains and the northern Great Basin into 
northern rockies... 

Widely scattered strong thunderstorms may impact parts of southern 
New England, the southern mid Atlantic coast, the Central High 
plains and parts of the northern Great Basin into northern rockies 
today, accompanied by some risk for severe weather. 

Troughing within a couple of branches of mid-latitude westerlies 
appears likely to remain in phase while generally developing 
eastward across Quebec, the northeast and mid Atlantic region during 
this period. Models suggest that there may be some further 
deepening of the primary surface cyclone associated with one 
embedded perturbation, as it migrates eastward near/north of the St. 
Lawrence Valley. A weaker secondary wave likely will be in the 
process of migrating northeast of the New England coast by the 
beginning of the period, ahead of a cold front forecast to advance 
through much of the northern and middle Atlantic Seaboard by 12z 

Upstream, models indicate that a mid/upper closed low will gradually 
evolve and deepen within lingering southern branch troughing, which 
is forecast to gradually accelerate eastward across the northern 
Great Basin into northern rockies. At the same time, a number of 
convectively generated or enhanced perturbations are forecast to 
continue to migrate around the western/northern periphery of a 
prominent subtropical ridge centered over the Southern Plains. 

..northern Great Basin into northern rockies... 
Beneath the mid-level cold pocket associated with the evolving upper 
low, models indicate the development of rather steep lower/mid 
tropospheric lapse rates with daytime heating. It appears that 
boundary layer moisture might be sufficient to support at least 
pockets of cape near or excess of 1000 j/kg as early as late 
morning, from the Wasatch and Uinta Mountains of Utah northward 
through the Absaroka Mountains of northwest Wyoming/southwest 
Montana. This may coincide with sufficient strengthening of deep 
layer shear to support organized convective development, including 
supercells, with at least a risk for severe hail and wind. Due to 
lingering uncertainties concerning the extent of the destabilization 
and potential convective coverage, severe probabilities are being 
maintained at 5 percent for now, but this may need upgrading in 
subsequent outlooks. 

..southern New England... 
Winds in the 850-700 mb layer may remain rather modest in strength 
across much of the region, in between one belt of stronger flow 
developing northward into Quebec in association with the primary 
cyclone, and another continuing to progress into the western 
Atlantic associated with the secondary wave. In association with 
the latter feature, seasonably high precipitable water content may 
also be in the process of advecting offshore early in the period. 
Lower/mid tropospheric lapse rates also appear likely to remain 
fairly weak, with colder air aloft generally lagging to the west of 
the surface cold front. 

However, guidance is suggestive that as pre-frontal surface dew 
points linger near 70f, surface heating could still contribute to 
cape on the order of 1000 j/kg by this afternoon. With the approach 
of the mid-level trough, this, coupled with favorable deep layer 
shear beneath 30-50 kt southwesterly mid-level flow, may be 
sufficient to support a couple of strong storms accompanied by a 
risk for marginally severe hail and strong surface gusts. 

..southern mid Atlantic coast... 
Models indicate that moderately large cape will develop by this 
afternoon, generally near the coast, across the coastal plain, in 
the vicinity of a pre-frontal confluence zone/sea-breeze. This may 
provide a focus for vigorous thunderstorm development, mainly this 
evening, aided by forcing for ascent associated with an approaching 
short wave digging into the base of the larger-scale mid/upper 
troughing. Shear beneath modest west/southwesterly mid/upper flow 
could enhance activity, which may be accompanied by a risk for 
potentially damaging wind gusts. 

..Central High plains... 
Mid-level lapse rates may become rather modest, resulting in cape 
only on the order of 1000 j/kg with heating by late this afternoon, 
focused along and south of a remnant convectively reinforced 
boundary north through east of the Raton Mesa area. However, models 
indicate that a nocturnally strengthening southerly low-level jet 
(30-40 kt) may impinge on this boundary and provide support for 
vigorous thunderstorm development, which may pose some risk for 
strong surface gusts this evening. 

.Kerr/leitman.. 08/22/2018 


Mesoscale Discussion

acus11 kwns 212307 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 212306 

Mesoscale discussion 1339 
National Weather Service Storm Prediction Center Norman OK 
0606 PM CDT Tue Aug 21 2018 

Areas affected...southeast PA...the WV Panhandle...northern Virginia...and 
parts of Maryland 

Concerning...Severe Thunderstorm Watch 346... 

Valid 212306z - 220030z 

The severe weather threat for Severe Thunderstorm Watch 346 

Summary...an isolated strong, locally damaging wind gust is 
possible, mainly across far northern Virginia into northern Maryland the next 
hour or so. Otherwise, severe threat is expected to quickly diminish 
this evening. 

Discussion...convection across the region has generally been 
sub-severe late this afternoon and weakening is expected into the 
evening hours. Some stronger cells were evident across northern MD, 
likely tapping into a local maximum in instability and shear. Some 
strong to damaging wind gusts will continue to be possible with this 
bowing segment the next hour or so. However, poor lapse rates, 
quickly decreasing low-level shear and loss of daytime heating 
leading to an increasingly stabilizing boundary-layer will limit 
persistence of near-severe level storms. Given expected trends, a 
downstream watch is not expected. 

.Leitman.. 08/21/2018 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 39587904 39657828 39817796 40347766 40647736 40727681 
40527577 39997558 39037607 38507693 38287823 38367922 
38737949 39587904