Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 271704
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1104 AM MDT Thu Jul 27 2017

.UPDATE...
Issued at 1054 AM MDT Thu Jul 27 2017

A threat for severe thunderstorms will exist later today for areas
along/east of the Laramie Range in southeast WY. Low-level stratus
has cleared, allowing for strong surface heating within an already
moist/unstable air mass characterized by dew points in the 60s for
many areas. The latest SPC Mesoscale Analysis shows CAPE values of
1500 J/kg along the I-25 corridor as of 16z, and this may increase
further with forecast soundings showing some cooling around 500 mb
over the next few hours. Mean H5-H3 flow around 30-40 knots should
yield sufficient shear for hail-producing supercells. Low LCL hgts
and low-level veering may present a non-zero risk for a tornado as
well. We went ahead and added mention of severe thunderstorms with
the recent upgrade to SLGT Risk from SPC.

&&

.SHORT TERM...(Today through Friday)
Issued at 330 AM MDT Thu Jul 27 2017

The potential exists for some strong to severe storms over the next
two days, especially for areas along and east of the Laramie Range.
A moist airmass will remain in place across the CWA with PW values
from 0.75 inches (west) to between 1.0-1.25 inches (east).  Today is
the second day after the fropa, and with better warming compared to
yesterday, good instability will develop by this aftn over the.
plains of southeast WY.  With dewpoints of 55-60F, both the GFS and
NAM show CAPE values rising to 1250-1500 J/kg. Instability drops off
quickly across the far eastern zones.  Bulk shear increases to
around 40 kts by late aftn as a shortwave trough moves into western
WY and westerly winds aloft come up.  The strongest winds aloft and
shear will exist over Converse/Niobrara counties from 21-00Z where
the NCAR Ensembles have the solid area of higher updraft helicities.

Thus, could definitely see some rotating storms over the plains of
southeast WY, and with the marginal risk from SPC, went ahead and
added small hail and gusty winds.  While brief heavy rain will be
possible due to the high PW values, current thinking is that the
stronger steering flow should limit flash flood threat overall. Kept
20-40 percent PoPs going into the late evening and early night over
Converse/Niobrara counties and the northern Panhandle as the
shortwave moves into northeast WY.  The pattern on Friday will be
similar to today, with temps rising another 5 degrees or so. The
risk of stronger storms will extend eastward into the western
Nebraska Panhandle with CAPE values up to around 1500-1750 J/kg off
the NAM.  Southeast sfc winds veering to northwest aloft will
produce decent deep layer shear.  The threat of storms to the west
of the Laramie Range will generally be less than the last few
days.

.LONG TERM...(Friday night through Wednesday)
Issued at 330 AM MDT Thu Jul 27 2017

Models continue to be in relatively good agreement with the synoptic
pattern through early next week. Similar to previous guidance, the
upper level high over the southern Great Plains will weaken late in
the weekend and will restrengthen over the Great Basin next week.
Jet level flow will remain about 60kts with middle level shortwave
features moving overhead. Will see remnant convection continue
Friday night as a shortwave trough moves overhead, with another
round anticipated on Saturday as additional midlevel energy and a
sfc boundary move across the region. PW`s will remain quite high
between 1 to 1.5" through this time, so still think there is
potential for heavy rains and flooding.  We begin the transition to
more northwest flow with drier air filtering in from the northwest
on Sunday. The threat for locally heavy rain on Sunday should be
more focused along upslope favored locations in the mountains and
near the Colorado border. Monday through Wednesday will feature
continued chances for showers and afternoon thunderstorms due to
shortwaves moving overhead in NW flow, with the threat for heavy
rain on Monday less due to lower PW in place. This will be
short-lived though as higher moisture looks to return to the CWA on
Tuesday and Wednesday in deep upslope flow. Temperatures on the
whole will be around normal for this time of year.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1154 PM MDT Wed Jul 26 2017

A few showers linger across east-central Wyoming at this hour
which should persist for another hour or two as a shortwave moves
across Wyoming. Stratus has already developed across the plains
and some of the higher valleys out west, although cigs are still
VFR at this time. Do think those will become IFR overnight for the
plains terminals as temps cool within this moist upslope
environment. Stratus is expected to burnoff through the late
morning with afternoon/evening showers and thunderstorms possible.


&&

.FIRE WEATHER...
Issued at 330 AM MDT Thu Jul 27 2017

Minimal fire weather concerns through Friday.  Minimum afternoon
humidity values will remain mostly above 25 percent and winds will
be relatively light.  Scattered thunderstorms will be possible
during the afternoon and evening.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...CLH
SHORT TERM...ZF
LONG TERM...RJM
AVIATION...RJM
FIRE WEATHER...ZF



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