Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 290956
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
356 AM MDT Thu Jun 29 2017

.SHORT TERM...(Today through Friday Night)
Issued at 355 AM MDT Thu Jun 29 2017

Weak front will pass through the area from the north this
morning. A much stronger shortwave and its associated front will
cross the region from the northwest this evening and tonight.

In terms of sensible weather, today will initially be cooler and
more stable east of the Laramie Range with the passage of the weak
boundary.  Boundary layer flow will quickly back more to the
northeast through the afternoon and with the approach of the
stronger front from the north late in the day, expect showers and
thunderstorms to develop especially over our northern counties where
low level convergence will be maximized. West of the Laramie Range,
guidance has trended a bit lower with sfc dewpoints and forecast
soundings once again indicate a deeply mixed boundary layer with
plenty of dry air, perhaps aided by a bit of subsidence in the left
rear quad of the upper jet over N CO.  Thus have added a Red Flag
Warning once again for the Upper North Platte Valley where fuels are
ready.

Tonight, models have trended stronger and slower with the upper
shortwave and associated front.  Steepening lapse rates aloft
coupled with good dynamic lift and a sfc front will keep showers and
thunderstorms going through the night, slowly sinking southward and
eastward with the front.  Could be some much needed rain especially
east of the Laramie Range.  Some small hail is possible as freezing
levels lower but the severe threat should be low due to unfavorable
diurnal timing.

Friday may end up being more cloudy and cool east of the Laramie
Range with northerly flow and some residual dynamic lift from the
slowly departing trough. Could be enough instability for some weak
convection over the mountains but should not see too much activity.

.LONG TERM...(Saturday through Thursday)
Issued at 355 AM MDT Thu Jun 29 2017

A dry day is in store for Saturday as a dry air mass combines with
shortwave ridging. Compared to 24 hours ago, the latest models have
trended a tad stronger and slower with the shortwave energy progd to
move thru the Rockies on Sunday and potentially even lingering into
Monday/Tuesday. At this point, it looks like thunderstorm chances on
Sunday will be confined to the Nebraska Panhandle during the
afternoon and evening hours. The latest GFS suggests the potential
for severe weather with SBCAPE values in excess of 1500 j/kg along
with adequate deep layer shear. The shortwave trough slows as it
descends the east side of the amplifying upstream ridge
Monday/Tuesday. This is the first indication that thunderstorm
chances may need to be added into Monday and Tuesday afternoons, but
will await the confirmation of subsequent model output before
adding. Expect Wednesday to be dry as warm mid-level temperatures
beneath the upper ridge suppresses convection. Afternoon/evening
thunderstorm chances to return on Thursday as PACNW upper-low
flattens the ridge. Highs will warm into the 80s through the period,
with even some low 90s across lower elevations for next week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 355 AM MDT Thu Jun 29 2017

VFR conditions to prevail at area terminals thru at least the
early afternoon hours. Another round of showers/thunderstorms will
develop after 21Z, initially mainly impacting areas nr KCYS,
before spreading to western Nebraska Panhandle airports after 00z.
For now, confidence is not high enough to add more than a VCTS
mention with this timing in mind. There is some indication that
weak convection may linger after 06Z as well. For now confidence
is not high enough in placement to add. Will re-evaluate with new
model information for the 18Z TAF issuance.

&&

.FIRE WEATHER...
Issued at 355 AM MDT Thu Jun 29 2017

Elevated conditions once again west of the Laramie Range this
afternoon with Red Flag conditions likely in FWZ 306 where fuels
are ready. RH values will be between 10 and 15 percent with gusty
west winds. After today, a cooler and more stable airmass will
move into the area so fire weather concerns Friday through
Saturday should be minimal.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Red Flag Warning from noon today to 6 PM MDT this evening for
     WYZ306.

NE...None.

&&

$$

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



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