Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 200640 AAA
AFDCYS

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1240 AM MDT Sun May 20 2018

.UPDATE...
Issued at 731 PM MDT Sat May 19 2018

Webcams between Cheyenne and Laramie along Interstate 80 are
already showing limited visibilities and fog. With an upslope
easterly wind expected to become southeast overnight and with
recent heavy rainfall across the area, expect this fog to become
quite dense tonight. In addition, would not be surprised if the
fog spread east of Interstate 25 later on tonight. Will issue a
Dense Fog Advisory for just the I-80 Summit and foothills for now,
but may need to extend the advisory eastward since Cheyenne is
beginning to observe fog as well.

**Previous update: Issued at 527 PM MDT Sat May 19 2018**

Current KCYS radar loop shows precipitation rates decreasing over
the area in the last few hours. Will let the Flood Watch expire on
time since the threat for additional flooding is highly unlikely.
However, will keep the Flood Warnings going through this evening
and assess the situation over the next few hours.

&&

.SHORT TERM...(Tonight - Sunday)
Issued at 241 PM MDT Sat May 19 2018

We continue to receive reports of significant flooding across much
of southeast WY and the western NE Panhandle this afternoon. Areal
Flood Warnings remain in place until 345 PM MDT, and will probably
require extensions into the evening per recent discussion with law
enforcement and emergency management officials. In general, waters
have been receding, but many creeks are still at bankful with more
rain possibly worsening the situation over the next few hours. The
area around Scottsbluff has been the hardest hit, with recent rain
totals of over 9 inches reported near Gering! Much of the area has
received over 4 inches of rain per radar estimates.

The intense/stalled low pressure system will slowly drift eastward
across northwest KS over the next few hours. A large area of light
to moderate rain continues to pinwheel around the system, w/ not a
lot of relief in sight in the very near term. Eventually, some low
level drying should take place as high pressure builds in from the
north. As such, expect to see a gradual decrease in rainfall after
00z, generally from west to east. Some fog is possible from the I-
80 Summit eastward into Cheyenne between 09z-15z with residual low
level moisture and favorable winds as rain ends.

A few pieces of mid-level energy should yield a few afternoon rain
showers or thunderstorms again on Sunday, mainly along and west of
the Laramie Range.

.LONG TERM...(Sunday night through Saturday)
Issued at 357 AM MDT Sat May 19 2018

An active first half of next week remains likely with several more
rain and storm chances into at least Wednesday. Models are in
better agreement in the longer range but unfortunately suggest a
semi-active northwest flow pattern towards the second half of the
week. Models diverge into the weekend with lower forecast
confidence.

Another large western CONUS closed low and associated trough axis
appears to become established by Monday. This will place the
region once again in unsettled SW flow with several H5-H3
perturbations transiting over WY. Diurnally driven showers are
likely Monday and Tuesday with a larger complex possible into
Wednesday morning depending on shortwave trough positioning.
Drier and cooler conditions will be likely Thursday as northwest
flow become established. Weak instability each afternoon could
allow for some shower and thunderstorm generation. However, the
overall coverage should be reduced from earlier in the week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1228 AM MDT Sun May 20 2018

IFR and LIFR conditions are ongoing near CYS with dense fog and
low ceilings occurring towards the Laramie Range foothills and
summit. MVFR to VFR conditions are present for the NE high plains
with VFR at LAR/RWL. Patchy MVFR ceilings likely overnight for
southern NE at SNY but sites farther north should remain VFR. CYS
will slowly improve to MVFR then VFR through the day Sunday.
Surface winds will be light and variable through the overnight and
then increase 10-15 knots Sunday from the south to southeast.

&&

.FIRE WEATHER...
Issued at 241 PM MDT Sat May 19 2018

Moist conditions will continue through tonight, especially for
areas along and west of Interstate 25 where widespread rain and
fog will be noted through late tonight.  Drier conditions expected
Sunday into Monday, however winds and humidity will remain above
critical levels through Monday.  The next chance of rain
comes Tuesday into Wednesday as chances of showers and thunderstorms
increase.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Dense Fog Advisory until 9 AM MDT this morning for WYZ116-117.

NE...None.
&&

$$

UPDATE...TJT
SHORT TERM...CLH
LONG TERM...JSA
AVIATION...JSA
FIRE WEATHER...JG



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