SPC Jul 25, 2024 2000 UTC Day 1 Convective Outlook

[NorthGeorgiaWX]

SPC 2000Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0259 PM CDT Thu Jul 25 2024

Valid 252000Z - 261200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PARTS OF CENTRAL
AND NORTHEASTERN MONTANA...PORTIONS OF NORTHEASTERN NEVADA AND
NORTHWESTERN UTAH...

...SUMMARY...
Scattered severe thunderstorms capable of 60-75 mph gusts are
forecast across portions of central/northeast Montana and
northeastern Nevada/northwestern Utah late this afternoon into the
evening.

...20z Update...
A Slight Risk was introduced across northeastern Nevada and
northwestern Utah, where scattered to numerous thunderstorms posing
a risk of severe gusts should develop by the afternoon. Recent runs
of the HRRR, as well as guidance from the 12Z HREF, continue to
indicate the possibility of convection consolidating along outflow
and surging northeast into northwestern Utah through the evening.
Forecast sounding profiles support risk for severe gusts, with steep
low to mid-level lapse rates and characteristic inverted-v
indicative of large dew point spreads. This will support downward
momentum transport of severe wind up to 60-70 mph, especially where
outflow storms can consolidate along outflow.

The Marginal Risk across Arizona was expanded westward into far
southwestern Arizona to encompass potential for a cluster of
thunderstorms to push westward by late evening. Thunderstorm
activity is expected to continue across the high country before a
gradual shift into the low deserts by the evening. Some
consideration was given to a Slight Risk in this area but
uncertainty remains in storm coverage along the Mogollon Rim and
whether strong outflow will develop and move southwestward into the
low terrain this evening. 

Otherwise, the eastern US Marginal Risk areas remain unchanged.
Storms capable of locally damaging winds will continue to be
possible through the afternoon and evening.

..Thornton/Smith.. 07/25/2024

.PREV DISCUSSION... /ISSUED 1131 AM CDT Thu Jul 25 2024/

...Montana/Northern Rockies to Great Basin...
Water-vapor imagery this morning shows an upper-level trough moving
east from British Columbia toward Alberta/Saskatchewan, with
strengthening cyclonic westerlies overspreading the northern
Rockies/Montana.  Model guidance continues to indicate scattered
thunderstorm development initiating off the higher terrain this
afternoon, and in advance of a cold front across central Montana. 
Forecast soundings shows large to very large surface
temperature-dewpoint spreads (40-60 F) later this afternoon.  A
downward mixing of 30-40+ kt ambient southwesterly to westerly flow
in the 700-500 mb layer may contribute to severe-caliber wind gusts
in consolidating outflows, as convection spreads eastward into this
evening toward eastern Montana and possibly western North Dakota.

Farther south over the Great Basin, model guidance appears to be
trending towards greater storm coverage (numerous) and associated
ability for storm outflow.  The 12 UTC HREF in particular (and
recent HRRR runs) shows a potential corridor of outflow surging
convection from northeast NV into northwest and northern UT during
the 23-04 UTC period.  Some consideration for a Slight Risk was
given but uncertainty precludes an upgrade at this time.  

...Arizona...
Lingering early day outflow/cloud cover via yesterday's relatively
active convection should generally abate by afternoon. As the
boundary layer diurnally warms and destabilizes, thunderstorms are
again likely to initiate across the Mogollon Rim and adjacent
mountains of southwest New Mexico this afternoon. Given the position
and movement of the mid-level high, somewhat stronger (around 15 kt)
northeasterly steering flow will remain focused across southeast
into adjacent portions of central Arizona.  Severe gusts will be
possible later this afternoon into the mid-late evening as storms
progress south-southwestward across the lower deserts of central and
southern AZ.

...Carolinas/Mid-Atlantic States...
Midday visible-satellite imagery shows and extensive cloud shield
across much of NC into the southern half of VA.  The 12 UTC GSO raob
featuring poor lapse rates and meager buoyancy contrasted with the
MHX raob (2000 J/kg MLCAPE) and a much more moisture-rich airmass. 
Thinner cloud cover near the coast will aid in widely scattered
storms developing this afternoon largely removed from the southern
fringe of large-scale troughing over the Mid-Atlantic
states/Northeast.  An isolated risk for locally severe gusts may
develop for several hours this afternoon into the early evening.

Farther north, breaks and thinning of cloud cover will promote
stronger heating across northern VA into the northern half of the
Chesapeake Bay vicinity than areas farther south.  Widely scattered
storms are forecast to develop per the latest 12 UTC HREF guidance
within this corridor by mid-late afternoon.  A belt of 40-50 kt
southwesterly 500-mb flow may enhance storm vigor, and a few
downdrafts may yield a risk for isolated 45-60 mph gusts capable of
localized damage.

...Upper Texas Coast and Southwest Louisiana...
Extensive showers and some thunderstorms are ongoing along the Texas
coast early today.  Influenced by a weak mid-level disturbance, some
guidance suggests that a strengthening of low-level winds/shear may
occur.  However, mid-level winds will remain weak, while the
aforementioned convection should considerably limit destabilization
inland. These factors seem likely to minimize the potential for
anything beyond a very weak/transient circulation.

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