- Day Three
acus01 kwns 240555
Storm Prediction Center ac 240554
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
1254 am CDT Thursday Jul 24 2014
Valid 241200z - 251200z
..there is a slight risk of severe thunderstorms this afternoon into tonight across
parts of eastern Montana into western and central North
Dakota...perhaps parts of adjacent northern South Dakota...
A few severe thunderstorms capable of producing large hail and
strong wind gusts are possible across parts of eastern Montana into
western and central North Dakota this afternoon into tonight.
Strong storms...with potential for localized damaging wind
gusts...may also develop across parts of the central and southern
Appalachians into the southern middle Atlantic coast states today.
The significant short wave trough and embedded closed low now over
the Pacific northwest appear likely to continue slowly inland during
this forecast period...as an upstream impulse progresses across the
northeast Pacific...toward the British Columbia coast. With
subtropical ridging currently prominent across much of the
intermountain west/rockies and plains...the lead system is forecast
to turn northeastward around its crest...across the Canadian and
northern U.S. Rockies...into Alberta and Saskatchewan. Models
suggest that substantive middle-level height falls will generally be
confined to Montana and portions of the Dakotas...with only minor
weakening of the subtropical high...although its center may shift
southeast of the central rockies into the southern High Plains.
Otherwise...little change to the large-scale upper pattern is
expected...with amplified troughing persisting east of the
Mississippi Valley through the Atlantic Seaboard. The leading edge
of low-level cooling/drying associated with this feature is expected
to advance southeast of the Ohio Valley into the South Atlantic and
Gulf Coast states. At the same time...a southerly return flow will
contribute moistening across the central into northern
plains...ahead of the northwestern upper troughing. However...this
is generally expected to occur beneath very warm and capping
elevated mixed layer air advecting northeast and east of the
northern and central rockies.
Highest probabilities for thunderstorms during this period appear to
exist ahead of the front...across the southeastern u... well as
within a plume of monsoonal moisture from the southern plateau
through the central rockies. But at least scattered thunderstorm
activity is expected within a Lower/Middle tropospheric warm advection
regime developing northeast of the middle Missouri Valley...as well as
ahead of a cold front advancing into the western Dakotas by late
..northern and Central Plains...
Stronger middle-level height falls are not expected to reach eastern
Montana and the western Dakotas until at least late this
afternoon...perhaps this evening. And very warm elevated mixed
layer air probably will suppress vigorous convective development
within the corridor of higher boundary layer moisture content along
the pre-frontal surface trough across the region. However...the
very steep middle-level lapse rates associated with this air mass will
contribute to sizable cape...and a rather broad area with at least a
conditional risk for severe storms.
It currently appears that...through much of the afternoon
hours...any thunderstorm activity will be widely scattered in
nature. Activity probably will form across the higher terrain
near/west of the north Central High plains...in response to daytime
heating and orographic forcing. But isolated/widely scattered storm
development may not be out of the question within a zone of enhanced
Lower/Middle tropospheric warm advection...near a southerly 850 mb
jet...on the eastern edge of the elevated mixed layer air across the
eastern Dakotas into Minnesota/Iowa.
Otherwise...it appears possible that forcing along/ahead of an
eastward advancing cold front could support storm initiation across
parts of eastern Montana as early as 21-23z. Although the stronger
middle/upper flow associated with the approaching upper trough may lag
to the west of the front...pre-frontal vertical shear across eastern
Montana into North Dakota still appears sufficient for organized
convective development...including supercells...given expected mixed
layer cape of 2000-3000+ j/kg.
The evolution of a mesoscale convective system capable of producing
one or two swaths of potentially damaging wind gusts seems at least
a possibility as upper forcing spreads eastward into the northern
plains this evening.
..cntrl/southern Appalachians into southern middle Atlantic coast region...
Weak Lower/Middle tropospheric lapse rates probably will limit cape
along/ahead of the southward advancing frontal zone today.
However...in the presence of seasonably high moisture
content...modest westerly middle-level flow may enhance convective
development...supporting at least low probabilities for storms
capable of producing potentially damaging wind gusts.
acus11 kwns 240741
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 240741
Mesoscale discussion 1455
National Weather Service Storm Prediction Center Norman OK
0241 am CDT Thursday Jul 24 2014
Areas affected...parts of eastern Montana into northwestern ND
Concerning...Severe Thunderstorm Watch 437...
Valid 240741z - 240945z
The severe weather threat for Severe Thunderstorm Watch 437
Summary...a band of storms -- including embedded strong/severe cells
-- continues moving east-northeastward across northestern Montana...within ww 437. Isolated
severe risk may spread eastward into northwestern ND over the next few
hours...where need for a new ww remains uncertain.
Discussion...latest radar imagery shows a band of strong/locally
severe storms moving across Phillips/Garfield counties and into
Valley County Montana at this time...with the strongest storms now on the southern
edge of the band /over Garfield County/. Damaging winds and large
hail remain possible with these storms -- particularly the
newly-evolving Garfield County cells.
The storms -- slightly elevated at this time above a stabilizing boundary
layer -- are moving through the axis of greatest mixed-layer cape
at this time /per objective analyses/. That said...ample elevated cape
still resides downstream into ND. Given ongoing warm advection
associated with a southerly low-level jet supporting background ascent
across this area...isolated severe storms /capable of producing
large hail/ may indeed survive/spread across northwestern ND in the next
several hours -- which could require downstream ww consideration.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 46880737 49000867 49040412 48910212 48370060 47960048
47550090 47550209 47370484 46900580 46880737