Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 121607

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1007 AM MDT Sat Aug 12 2017

Issued at 950 AM MDT Sat Aug 12 2017

Main source of concern today will be the potential for severe
thunderstorms over the northeast plains. RAP/NAM12 forecast
soundings show very high CAPE values toward Sterling late this
aftn and evening, 3000-4000 j/kg which may be overdone but hard to
ignore. CAPES high even if those values are cut in half. Forecast
sounding pretty high as far south as Lincoln County with CAPES of
2000-2500 j/kg around 00Z. For this reason will expand the
mention of severe storms into Lincoln County. Main area of concern
along and east of a line from around New Raymer to Ft Morgan to
Simla. Highest boundary layer theta-e values will be along and
east of there. Large hail and damaging winds the main threat but
could not rule out the possibility of a tornado. To the west much
drier in the lower levels with CAPES in the 500-800 J/KG range
this afternoon and evening.


.SHORT TERM...(Today and tonight)
Issued at 322 AM MDT Sat Aug 12 2017

Northwest flow aloft will prevail over the Central Rockies
through tonight. HiRes models showing an increase in thunderstorm
activity today versus yesterday over the higher terrain and along
the Front Range. This is likely due to better heating leading to
increased instability. A short wave trough over southwest Montana
will move southeast across Wyoming today. The best forcing with
this wave will stay north of Colorado. However, models show
thunderstorms clipping far northeast Colorado early this evening.
Will have likely pops for this. Otherwise, expect isolated to
scattered convection today. Airmass over the higher terrain and
along the Front Range Urban Corridor is somewhat dry with
precipitable water values of three- quarters of an inch to one
inch. Mixing drier air down will drop dew points into the 40s this
afternoon. This produces surface based CAPE of less than 700
J/kg. Over the far eastern plains, the airmass is expected to have
more moisture with dew points staying in the 50s. This leads to
surface based CAPE values of 1000-2000 J/kg. The best chance for
severe storms will be over Logan/Washington counties and eastward.
Golf ball size hail and damaging winds to 80 mph will be possible
with the severe storms.

Due to a warmer airmass and increased sunshine, most locations
across northeast Colorado are expected to climb into the lower to
mid 80s this afternoon. Temperatures tonight should also be
slightly warmer with mid to upper 50s. Storms are expected to end
mid to late evening as they push east/southeast out of the area.

.LONG TERM...(Sunday through Friday)
Issued at 322 AM MDT Sat Aug 12 2017

Operational models and a majority of the ensembles show the 500-
300mb ridge axis over Utah Sunday morning. Models also indicate
a fairly strong northwest mid-level flow crossing Wyoming and
northeastern Colorado, and within this flow a long fetch of mid
and high-level moisture extending back to the Montana Rockies.
This moisture together with modest lift generated by a nearby jet
should be adequate to produce a few light rain showers across the
nern corner of the CWA to start the day. Guidance shows the upper
ridge slowly migrating eastward over Colorado during the day and
reaching the western Great Plains by Monday morning. With its
passage, look for the mean layer flow to gradually transition from
northwest to southwest. Ordinarily such a shift in flow would
mean warmer temperatures. However, monsoonal moisture carried
along by this flow should keep afternoon and evening skies partly
cloudy to overcast and the possibility of t-storms in the
forecast for most areas through Tuesday. Storm chances appear
to creep up Sunday and Monday, and by Tuesday afternoon peak
esply on the northeast plains and eastern tip of the Palmer Divide
where low-level moisture, convergence and deep instability
will interact with a passing shortwave trough to produce scattered
thunderstorms a few of which may turn severe across the northeast
corner of the CWA. Large hail will likely be main threat, but
cannot rule out an isolated brief tornado ahead of a weak cold
front swinging down from Wyoming. Overnight, most of this storm
activity will exit the region as drier, stable air advects in
behind the front and departing shortwave trough.

By Wednesday, the CWA will lie at the bottom of a broad upper
trough draped over the northern tier state. And, with it the flow
aloft goes zonal. Models still indicate a fair amount of moisture
contained in this flow, and therefore with strong daytime heating
we should again see scattered t-storms in the high country and
mainly isolated storm coverage on the plains from mid afternoon
through early evening. PW values will be down, so rainfall
amounts will be lower. Temperatures on Wednesday are expected to
be similar to those predicted earlier in the week.

For Thursday and Friday, models generally agree on rebuilding the
Great Basin upper ridge as the upper trough mentioned above
shifts east over the northern Great Plains and Great Lakes states.
This will likely result in lower humidity, slightly warmer
temperatures and a marked reduction in t-storm activity for the
CWA both days. May even see max temps on the plains of nern
Colorado reaching the 90 deg mark by Friday afternoon.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 950 AM MDT Sat Aug 12 2017

VFR conditions through tonight, but could see brief ILS
restrictions during the passage of thunderstorms this aftn and
early evening. Could see bkn cigs 050-060 kft agl at that time.
The best window for thunderstorms at the Denver airports will be
21Z to 01Z. Brief heavy rain, small hail, and wind gusts to 40
knots will be possible with the stronger storms.




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