000
FXUS65 KCYS 150659
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
Issued by National Weather Service Boulder CO
1259 AM MDT Mon Apr 15 2024

.KEY MESSAGES...

- Critical fire weather conditions are possible across the
  southern Nebraska Panhandle Monday where Red Flag Warnings are
  in effect from noon to 7 PM for gusty winds and low humidity.

- The next storm system will arrive Monday afternoon and bring
  showers and thunderstorms followed by strong winds to portions
  of the area. Accumulating snow is expected in the mountains,
  and snow may mix in for the higher valleys and plains of
  southeast Wyoming Monday night into Tuesday.

- Strong cold front associated with a large system across
  central Canada will bring much colder temperatures and the
  chance for valley snows Wednesday into Friday.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 340 PM MDT Sun Apr 14 2024

You name it we got it in the upcoming days with critical fire
weather...strong winds...winter weather and possible severe
convection all possible.

Currently...Stationary boundary lays along the front
range...north to the Laramie Range to a 1008mb surface low near
Buffalo Wyoming and finally into eastern Montana this
afternoon. Fairly quiet right now across southeast Wyoming and
Nebraska Panhandle. Off to the west...a Pacific low pressure
system has finally moved onshore across central California and
this will be the major contributor to our active weather
upcoming.

This low ejects inland overnight into southern Nevada and
eventually to central Utah by 12Z Monday morning. 700mb low
located across north central Colorado by mid afternoon Monday
with an almost vertically stacked 500mb low. Surface low located
just south of Sidney by 21Z Monday. Very dry air near the
surface low with 850mb winds 25-35kts out ahead of the low. With
the very dry air and strong winds...though it was prudent to
issue a Red Flag Warning for the southern Panhandle from noon to
7PM Monday.

To the northwest of this low...northwest winds forecast to
increase significantly Monday afternoon and evening. GFS
750/800mb winds up near 50kts across the Summit into Laramie
County Monday afternoon. GFS 700-800mb winds become stronger and
more widespread Monday evening into Tuesday morning. They peak
Tuesday morning near 55-60kts along and east of the Laramie
Range into the southern Nebraska Panhandle. Similar set up to
the wind storm we had a week or so ago. With that in
mind...decided to expand the High Wind Watches east into the
southern Panhandle.

Guidance consistent with increasing QPF and PoPs across our
western zones and especially the north facing slopes of the
Snowy and Sierra Madre Ranges. Getting anywhere from 17 to 24
inches in the mountains out west from 18Z Monday to 21Z Tuesday.
Decided to issue a Winter Storm Watch for the western
mountains.

Finally...there is a risk for severe thunderstorms Monday
afternoon and evening. HRRR and RAP simulated radar showing
convection continuing into the evening and past midnight across
our northern forecast zones into the northern Nebraska
Panhandle. NAM MUCAPE across Niobrara County into the Nebraska
Panhandle on the order of 1500-2000 J/KG from 21Z through the
evening hours. By far our best instability so far this spring.
Steep mid level lapse rates of 9C/KM across the Panhandle Monday
afternoon SPC maintains a Marginal Risk area from Sidney to
Torrington to just west of Lusk and a small sliver of Slight
Risk in Dawes County for Monday afternoon and evening. So can
not rule out a couple severe thunderstorms for Monday afternoon
and evening.

Upper low finally shifts east Tuesday afternoon with the wrap
around moisture shifting east as well. Upper level ridging
moves in from the west and we should see drying and warming
conditions by that time.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Issued at 340 PM MDT Sun Apr 14 2024

An extended period of active weather is expected for much of
southeast Wyoming and western Nebraska on the back side of the early
week system. Upper-level diffluence is progged to move overhead
Wednesday as the upper-level jet strengthens behind and swings in
from southern Canada. Zonal flow will develop late in the day as the
upper-level trough flattens out, placing the CWA underneath the
favorable right entrance region of the upper-level jet.
Precipitation chances will increase throughout the day Wednesday as
a 700mb cold front and an accompanying surface cold front push into
the region from the northwest. Some differences exist between the
long range models as to how much precipitation will fall with this
front. The GFS is a bit more bullish on precipitation for Wednesday
night into Thursday, while the ECMWF is slightly lower. The
differences appear to stem from the models suggesting that a surface
low will try to develop across southeastern Colorado during the day
Wednesday. The ECMWF suggests that this low develops and quickly
moves off to the east as the low is picked up by the upper-level
flow, decreasing the total precipitation expected. The GFS on the
other hand suggests that the surface flow attempts to develop, but
is slower. The surface low in the GFS develops before being pushed
south by the encroaching cold front and is flattened into a surface
trough rather than a closed low. The slower GFS solution would
result in more precipitation across the region, with the surface
trough sticking around longer than the more progressive ECMWF
solution. Despite these differences, both models suggest that after
another warm day Wednesday, colder temperatures will return for
Thursday onwards.

Colder air is expected to remain over the region Thursday through
most of the weekend as the CWA will be influenced by the broad,
upper-level trough of Canada. 700mb temperatures remain in the -5 to
-10C range once again, leading to daytime highs only in the 40s for
Thursday through Saturday. The region will be broadly within the
right entrance region of the upper-level jet for much of the end of
the week into the weekend, leading to continued synoptic support
across the region favoring the development of rain and snow shows
after the main push of showers from the cold front Wednesday night
into Thursday. In addition to this, several 500mb vorticity swaths
will traverse across the region leading to favorable cyclonic
vorticity advection to further support continued lift. Daily chances
for rain and snow showers are expected from Thursday throughout
Saturday with temperatures only reaching the 40s area wide.

A brief warm up is expected on Sunday as a relatively small, upper-
level ridge builds over the region ahead of the next incoming
system. The ridge will bring an end to the shower chances across the
region and allow temperatures to warm back into the 50s and 60s area
wide. Exactly how long this warmth will last is uncertain at this
time as the ECMWF and GFS suggest very different solutions. The GFS
suggests the upper-level ridge builds ahead of an incoming upper-
level low that is over the Pacific Northwest early Monday morning.
This system would be expected to bring cooler temperatures to the
region with the possibility of some precipitation. The ECMWF at this
time suggests no upper-level trough at all, instead suggesting the
upper-level ridge is the dominate feature that remains over the
western CONUS for several days. This will need to be monitored as
the ECMWF and GFS likely come into better agreement over the next
several days.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1259 AM MDT Mon Apr 15 2024

VFR conditions are expected across all terminals through the
valid TAF period. The main aviation concern will be the gusty
westerly winds over the next several hours, in which wind gusts could
approach 30 knots. There is an additional concern for some low
level wind shear issues across the Nebraska terminals between
09-13z across the Nebraska terminals.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from late Monday night through Tuesday
     afternoon for WYZ101-107-108.
     Winter Storm Watch from Monday afternoon through Tuesday
     afternoon for WYZ112-114.
     High Wind Watch from Monday afternoon through Tuesday
     afternoon for WYZ116-117.
     High Wind Watch from late Monday night through Tuesday
     afternoon for WYZ118-119.
NE...Red Flag Warning from noon to 7 PM MDT Monday for NEZ436-437.
     High Wind Watch from Tuesday morning through Tuesday evening
     for NEZ019>021-054-055.

&&

$$

SHORT TERM...GCC
LONG TERM...AM
AVIATION...TJT