- 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
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
..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.
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
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.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 39587904 39657828 39817796 40347766 40647736 40727681
40527577 39997558 39037607 38507693 38287823 38367922