Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 172049

Area Forecast Discussion
National Weather Service Denver/Boulder CO
249 PM MDT Thu May 17 2018

.SHORT TERM...(This evening through Friday)
Issued at 156 PM MDT Thu May 17 2018

Main concern is with severe potential later this afternoon/evening
over the far northeast plains and even greater threat over all of
plains Friday afternoon and evening.

Upper low is across east central Nevada this afternoon with a
generally dry southwest flow aloft over Colorado. For this
afternoon some weak high based convection trying to get going over
the mountains but the low levels are quite dry. There is a weak
convergence boundary moving north across central portions of adams
and arapahoe counties with much drier air to the south. This could
help focus of storms further north into Weld and morgan counties.
If storms can advance into the richer air across the far northeast
plains then a few severe storms are possible with large hail and
damaging winds through this evening.

Behind this evening convection there will be a return of low level
moisture later tonight under a weak northerly flow with
developing surface trof. This will set the stage for higher
surface based moisture on Friday. The upper low over Nevada today
will be moving into Western Colorado Friday afternoon with
increased widespread ascent ahead of the low. Could see severe
storms over much of the plains Friday afternoon with surface based
CAPES up to 2000j/kg with sufficient shear for supercells. Could
see large hail up to golf balls, heavy rain and damaging winds.
Temperatures will be cooler on Friday with increased cloud cover
and shallow upslope.

.LONG TERM...(Friday night through Thursday)
Iss.ued at 156 PM MDT Thu May 17 2018

By Friday night, the upper trough will extend over the four corners
region as the jet digs south. The surface low that was in place
over southern CO will transition North slightly bringing surface
winds more ESE just ahead of a frontal passage. Current model
trends have the front pushing south into the CWA between 03 and
06z late Friday night. CAPE values will continue to be elevated
over the eastern plains and some storms could continue into the
evening as the incoming boundary will help fire off some cells and
track eastward. The surface low will continue its NE trek into
central Kansas by 12z Saturday bringing winds around to a more
favorable NNE direction. By 12z Saturday specific humidity values
increase towards 4 g/kg which is on the higher side with PWAT
values between 0.75 and 1 inch in some areas on the plains. Models
are showing an extensive swath of moisture across most of NE
Colorado by Saturday morning that continues through the day. Model
cross sections show the moisture extending well into the
atmospheric column where it will bring a mix of showers and
thunderstorms to the high country where conditions could be less
stable for areas mainly west of the divide. The front will push in
completely by the morning hours on Saturday dropping 700mb
temperatures into the low single digits. With the cold push from
the North and increased upslope conditions some cold air damming
may be possible along the foothills. This cooler push could help
to drop snow levels to the 9500 mark bringing some snow to the
higher foothills and mountains east of the divide. Especially the
higher elevations of the northern Foothills where both the GFS and
NAM show decent upslope through most of the afternoon and evening
on Saturday. Some accumulation is possible with a possible 4 to 8
inches over the higher terrain and upslope areas. As for the
lower elevations rain will be steady into the early evening hours
on Saturday as the upper low transitions eastward into Nebraska.
Upper level winds will switch around to the NW and subsidence will
increase helping to bring an end to precipitation. Amounts will
vary but current model guidance shows at least 0.5 to 1 inch
possible with some areas seeing the 2 to 4 inches over the course
of a 24 hour period with the combination of thunderstorm heavy
rain and the steady stratiform rain from the front. Temperatures
will be below normal with highs only reaching into the low 50s on
the plains and 30s to low 40s in the mountains and mountain

For Sunday, conditions will clear out with some fog possible over
the plains during the early morning hours. By the late afternoon
hours instability increases over the higher terrain as the cap on
the plains will be hard to break. Some storms could initiate over
the higher terrain of the Palmer divide and push eastward but will
not survive long in the more stable atm. Temperatures will
rebound back into the mid 60s by the afternoon.

Into next week the upper low over California will close off and
track ESE. Ahead of the upper low flow will turn SW with an
embedded shortwave projected to push through the region Sunday
night into Monday. However, without the instability storm chances
are low. There continues to be a chance of showers and
thunderstorms during the afternoon hours through mid week with a
gradual warm-up into the low 80s by Wednesday.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 156 PM MDT Thu May 17 2018

Surface based convergence boundary moving north and generally
staying just east of KDEN. Another weak boundary coming off the
foothills should shift winds more nw at kden towards 22z. Could be
some weak high based convection over kden but should rapidly scoot
to the northeast. Will not mention any showers for now. still
could be some stratus later tonight and Friday morning but high
res models are backing off on this a bit. Better chance for storms
will be on Friday afternoon as the upper low approaches Western




SHORT TERM...Entrekin
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