SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0655 AM CST Wed Dec 16 2020

Valid 161300Z - 171200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
COASTAL NORTH CAROLINA...

...SUMMARY...
Severe thunderstorms are possible across parts of coastal North
Carolina, and the Gulf coastal areas of west-central/northwestern
Florida.

...Synopsis...
In mid/upper levels, the split-flow pattern -- now regulated by the
strong trough extending from eastern IA to south-central TX -- will
become more unified as the trough moves east and deamplifies
slightly.  Still, a potent shortwave trough should extend by 00Z
across portions of OH eastern KY/TN, to near CSG. The trough should
become even more compact and move over the NJ/NYC area between
09-12Z.

At 11Z, surface lows were drawn just south of the AL Coast, and over
Atlantic shelf waters southeast of CHS.  A warm front extended from
each low, southeastward to east-southwestward over waters of the
northeastern Gulf and Atlantic, respectively.  A secondary warm
front/marine-modification boundary, north of the synoptic front and
near the west edge of the Gulf Stream, extended from the Atlantic
low northeastward, parallel to the southeastern NC coast then east
of HSE near buoy 41001.  The latter boundary will move more slowly
until the main warm front overtakes it from the deepening low
eastward.

The Gulf low should move inland obliquely across the central/
eastern FL Panhandle today and weaken, while the Gulf warm-frontal
segment moves eastward and only gradually diffuses. Meanwhile the
Atlantic low will move north-northeastward over eastern NC and
evolve into the dominant, deepening cyclone center over the southern
Delmarva region by 00Z.  The main cold front will extend from there
across the FL coastal bend region and over the eastern Gulf, with a
combined warm front over Atlantic waters east of the Hampton Roads
region.  By 12Z, the primary low should be deeper and occluded
offshore from the Mid-Atlantic, with the cold front moving
southeastward over central FL.

...Eastern NC...
Scattered thunderstorms should move northeastward over the outlook
area this afternoon, offering threats for damaging gusts and a few
tornadoes.  The outlook follows the expected path of the warm
sector, east through south of the surface low across easternmost
portions of NC.  Boundary-layer theta-e in relatively convectively
uncontaminated parts of the warm sector should contribute to the
development of surface-based effective-inflow parcels with 500-800
J/kg MLCAPE.  This is most probable over the Outer Banks and sounds.
Favorable destabilization is more uncertain/marginal with westward/
inland extent, given more prior precipitation in the low-level
trajectories.

Deep-tropospheric wind fields should increase throughout the day as
the mid/upper trough and associated strong height gradients
approach.  This, along with backed low-level winds along and south
of the warm front, will yield favorable shear for supercells (e.g.,
effective-shear magnitudes 45-55 kt and effective SRH 150-300 J/kg,
in forecast soundings with surface-based parcels).  The threat
should ramp up around midday/18Z and diminish markedly after about
00Z.

...Coastal FL Panhandle to Tampa Bay region...
An ongoing area of precip -- with isolated, embedded, nonsevere
thunderstorms north of the surface warm front -- will cross the
eastern Panhandle and northern FL this morning through midday. 
Behind that, the northern and eastern fringes of a narrow,
marginally destabilized slice of warm-sector air will brush along
coastal areas of the central/eastern FL Panhandle, coastal bend, and
northwestern FL.  Greater inland penetration of favorably buoyant
air is possible this afternoon over parts of central FL.  In all
cases, weak deep-layer lapse rates near the decaying warm-frontal
zone should keep MLCAPE generally below 1000 J/kg.  Though this is
not optimal for FL severe, some onshore progression of supercells
and/or bowing segments from the Gulf may occur while shear still is
favorable, with a conditional threat for a damaging to severe gust
or tornado before activity weakens inland.

Prefrontal surface flow will veer through the afternoon, amidst
larger-scale mass response to the NC coastal cyclogenesis.  With
time, this gradually will reduce hodograph size and low-level vector
shear, and with that, supercell potential.  Destabilization should
be more marginal with eastward/inland extent across northern FL,
especially before the surface veering occurs to advect higher-
theta-e Gulf air farther inland.

..Edwards/Smith.. 12/16/2020

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