Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 120925

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

.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 and a fairly strong
northwest mid-level flow crossing Wyoming and northeastern Colorado.
Models also indicate a long fetch of mid and high-level moisture
extending southeast from the Montana Rockies to sern Wyoming and
nern Colorado. This moisture together with the modest lift provided
by a nearby jet may be adequate to generate a few light rain showers
across the nern corner of the CWA to start the day. Guidance show 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 result
in warmer temperatures. However, monsoonal moisture carried along by
this flow will 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 are expected to creep up Sunday
and Monday, and by Tuesday should see a peak in this activity esply
on the northeast plains and eastern tip of the Palmer Divide when
low-level convergence, instability and moisture interact with a pas
sing shortwave trough in southwest flow aloft to a few strong if not
severe t-storms across the northeast corner of the CWA. Large hail
will likely be main severe weather 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 departing shortwave

By Wednesday, the CWA should lie at the bottom of a broad upper
trough stretched over the northern tier state. And, as result the
flow aloft will become zonal. Models still indicate a fair amount of
moisture contained in this flow, and therefore with strong daytime
heating should again see scattered t-storms in the high country and
mainly isolated storm coverage on the plains mid to late afternoon.
PW values will be down, so potential precip amounts will be lower.
Temperatures on Wednesday are expected to be similar to those
observed 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 12Z TAFS through 12Z Sunday morning)
Issued at 322 AM MDT Sat Aug 12 2017

There are areas of low clouds to the north and east of the Denver
area, including over KDEN. Expect these clouds to linger through
mid morning. Will have a tempo for half mile visibility or lower
at KDEN through 11-14Z, when the best chance for fog is. West to
southwest winds will likely keep low clouds out of KAPA and KBJC.
Afternoon and evening thunderstorms will move across eastern
Colorado. The best time 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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