Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 271050
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
450 AM MDT Sun May 27 2018

.SHORT TERM...(Today - Monday Night)
Issued at 447 AM MDT Sun May 27 2018

An active 24-48 hours ahead for a large portion of southeast Wyo &
the western Neb Panhandle. Severe thunderstorms and flash flooding
are the primary items of interest this period. Morning Water Vapor
satellite imagery indicated a vigorous closed upper level low over
the Great Basin, with abundant mid/high level moisture streaming N
across eastern Colorado/Wyoming. The overall synoptic setup is not
likely to change much through late Mon, w/ strongly difluent south
to southwesterly flow aloft remaining in place ahead of the rather
slow moving/low-amplitude wave. A rather persistent moist conveyor
belt will help maintain deep moisture profiles, and likely support
widespread thunderstorms over the next couple of days. Some storms
are likely to become severe, and possibly produce heavy rain.

Surface cyclogenesis should take place over eastern Colorado today
as a modest 80-100 knot H25 jet noses into northwestern New Mexico
by 21z. Meanwhile, an inverted surface trough should develop north
along the Laramie Range in southeast Wyoming, with moist southeast
low-level flow taking shape to the east. Strong convergence, along
with moist upslope and plenty of dynamic support aloft will likely
support the development of numerous thunderstorms this afternoon &
evening. PWATs are projected to be high, but certainly not extreme
around 1 inch. However, we have concerns about deep southerly flow
parallel to the boundary in the presence of plentiful moisture and
tall/skinny CAPE profiles especially over northern areas. We could
very easily see training thunderstorms with heavy rainfall, and w/
concerns of Memorial Day recreation impacts we have issued a Flash
Flood Watch per coordination with WFOs Riverton and Rapid City. We
also have some concerns for severe convection, especially early in
the event (late afternoon/early evening) before things become more
messy. CAPEs in excess of 2000 J/kg and roughly 35-40 knots of 0-6
km shear may support some supercellular structures capable of hail
and wind. A couple of landspout tornadoes cannot be ruled out this
afternoon/early evening with significant low-lvl directional shear
along the surface boundary extending north and west across the CWA
later today. Biggest question today will be storm coverage w/ very
warm temperatures aloft over southern areas. The greatest coverage
seems likely over northern areas on the edge of the cap, but could
see a rapid transition to a heavy rain threat in those areas.

Thunderstorms will likely linger well into the night, and this may
have a significant impact on the mesoscale environment and overall
severe threat on Monday. We believe that Monday has more potential
to be a significant severe weather event, but lingering rain & low
clouds could significantly impact instability. Even so, the latest
GFS continues to nudge 60-65 dew points into western NE between 21
and 00z, supporting a moderately/strongly unstable air mass in the
presence of strong deep layer vertical shear on the order of 40 to
50 knots. Lower T/Td spreads on Monday suggests lower LCLs, so the
potential is there for a few tornadoes if clouds/precipitation end
early enough in the day. Otherwise, a conditional threat for super
cell thunderstorms capable of large hail and damaging winds should
exist over the high plains.

.LONG TERM...(Tuesday - Saturday)
Issued at 447 AM MDT Sun May 27 2018

Cool and unsettled conditions will linger on Tuesday as slow-moving
low-pressure lifts onto the high plains. Although ribbon of best
instability will be well east across central Nebraska, there may be
just enough lingering for one or two strong storms across the
southern Nebraska Panhandle. Temperatures warm back above seasonal
normals on Wednesday and will remain there into next weekend. Weak
transitory ridging on Wednesday & Thursday may be infiltrated by a
couple of showers and/or weak thunderstorms, but for the most part
these two days will be mostly dry. Thursday looks to be the warmest
day of the week, with highs warming into the 80s east of the Laramie
Range, with 70s west. A Pacific cool front approaches and most west-
east thru the region late Friday. Moisture will be limited with this
feature, so most likely sensible weather impact will be increasing
winds on Friday. Next weekend looks a tad cooler compared to the end
of the week, with highs warming into the 70s most places by Sunday.
Otherwise, a dry zonal pattern should keep any convection chances at
bay.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 430 AM MDT Sun May 27 2018

Main forecast challenge for this period is the timing of anticipated
afternoon/evening thunderstorm chances. Thunderstorms develop KRWL-
KLAR-KCYS between 19-21z. These storms will come to an end at KRWL-
KLAR by around 00z, but linger at KCYS until 03z. Thunder threat
will hold off until after 00z at Nebraska Panhandle sites.
Convective timing remains uncertain at this time, thus have left
VCTS mention in the TAFs. There is a threat of severe weather and
heavy rain with these thunderstorms this afternoon. It looks like
KCYS stands a fair chance for low stratus at the end of the period
as wind turns upslope from the northeast.

&&

.FIRE WEATHER...
Issued at 447 AM MDT Sun May 27 2018

No major fire weather concerns. One more hot day today, but plenty
of low-level moisture will keep RH values well outside of critical
thresholds. Thunderstorms are likely across much of southeast WY &
the western NE Panhandle this afternoon through Monday, giving way
to good chances for measurable precipitation for most areas.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Flash Flood Watch from noon MDT today through this evening for
     WYZ101>103-106>108.

NE...Flash Flood Watch from noon MDT today through this evening for
     NEZ095-096.

&&

$$

SHORT TERM...CLH
LONG TERM...CAH
AVIATION...CAH
FIRE WEATHER...CLH



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