119
FXUS65 KBOU 270937
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
337 AM MDT Tue Jun 27 2017

.SHORT TERM...(Today and tonight)
Issued at 337 AM MDT Tue Jun 27 2017

An upper level trough moving across the Northern Rockies will
push the upper level ridge over Colorado east of the state by this
evening. A surface low pressure trough/dry line will advance east
across the eastern Colorado this afternoon. Southerly winds ahead
of it will transport moisture northward with dew points in the
50s. West of the dry line, much drier conditions will prevail with
dew points falling into the 20s and 30s. CAPE ahead of the dry
line will reach around 2000 J/kg. So if storms are able to form,
they may become severe with golf ball size hail and damaging
winds. Biggest questions are when and where the storms form. The
dry line could progress through the area before convection is
triggered. The best chance for storms will be over the northeast
corner of the state where the moisture and instability will linger
the longest.

Gusty westerly downslope winds are expected behind the dry line
this afternoon along the Front Range. Expect very warm conditions
with temperatures topping out in the 90s. Red flag conditions will
likely be met where fuels are critically dry.

A cold front associated with the upper level trough will drop
south into northern Colorado this evening. Could be enough
moisture over the north central mountains for a few weak showers
or storms, otherwise it should be dry behind the front tonight as
it drops south across eastern Colorado.

.LONG TERM...(Wednesday through Monday)
Issued at 337 AM MDT Tue Jun 27 2017

While zonal flow remains aloft, a northwesterly low-level flow
will spread over northeast Colorado in the wake of a surface
trough which models show swinging down across the area during the
pre-dawn hours. It`s not quite a cold front, though it will usher
in cooler temps from Wyoming, resulting in max temps for the
plains 7-10 deg f cooler than those today. With the early passage
of this trough and a sfc high setting up in the Nebraska
panhandle, we should see the relatively dry nwly flow on the
plains turn sely during the afternoon, which directs a ribbon of
low-level moisture up across sern sections of the forecast area.
Model soundings indicate mean layer CAPES in the 1500-2200 j/kg
range in this area during the mid to late afternoon hours. The
latest Day 2 Convective Outlook from SPC has a slight risk of
severe t-storms in this area. Strong heating and convergence
should initiate storms in this area by around mid-afternoon, with
a few supercells capable of large, damaging hail and strong gusty
winds. Areas farther north should escape with only isolated gusty
high based storms moving off the Front Range late in the day.

Meanwhile, the mountains and high valleys will also get a slight
break from the heat on Wednesday with 700 mb temps cooling 2-3
deg C, while remain mostly dry. Even with the cooler temperatures,
the fire danger will remain elevated in the valleys of Summit and
Grand counties. For now, will hold off on any fire weather
highlights as winds are expected to lighter and min RH values a
shade higher.

On Thursday, models show the bottom side of an upper level trough
swinging down from Idaho/western Wyoming. As this trough passes,
the zonal flow aloft becomes northwesterly which models keep in
place through the end of the week. This wind shift appears to
drive a sfc cold front southward through nern Colorado Thursday
morning, lowering max temps another 3-6 deg f. The air behind
this front appears dry, however a weak shortwave disturbance in
the nwly flow aloft provides its own batch of moisture, mainly at
mid-levels. Upslope flow along the Palmer Divide coupled with
strong heating is expected to concentrate most of the late day
convection/t-storm development across the southeast fringe of the
CWA. However, with dewpoints in the 40s and marginal instability,
storms should be weak and be of short duration. Progressively
drier air filters down from Wyoming that evening, putting an end
to most of this activity. Same goes for the isolated poorly
organized storms in the high country.

On Friday, yet another weak cold front is forecast to brush nern
Colorado. Should see temps cool another 3-5 deg f, with highs
mainly in the upper 70s/lower 80s at lower elevations, and in the
60s/lower 70s across the high country. Could see a similar setup
for t-storms on the plains, though less available moisture will
likely result in fewer/weaker storms. Same goes for the high
country.

For the period Saturday through Monday...the resident airmass is
progged to gradually dry and warm as a large mid/upper-level
ridge builds in from the lower great Basin/Desert Southwest.
Should see temperatures start out around seasonal norms on
Saturday and end up 4-6 deg f above average for the date by
Monday. That means a return to lower 90s on the plains with
just a slight chance for late day t-storms.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday evening)
Issued at 337 AM MDT Tue Jun 27 2017

VFR conditions are expected through tonight. Main issue today may be
gusty west winds. Southerly winds this morning will turn southwest
around 15z and westerly around 18Z. Gusty west winds are expected
through 01Z with gusts to 30 knots possible. Winds will decrease and
turn northerly this evening.

&&

.FIRE WEATHER...
Issued at 337 AM MDT Tue Jun 27 2017

A fast moving upper level system and a dry line will advance east
across Colorado today. Behind the dry line, relative humidity will
fall into the lower teens. West winds will also increase with gusts
to 30 to 40 mph possible with the stronger winds over the higher
terrain. A few high based showers and thunderstorms will be possible
over the northern mountains today. Expect little rainfall and gusty
winds from these showers and storms.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon today to 9 PM MDT this evening for
COZ211-213-214.

&&

$$

SHORT TERM...Meier
LONG TERM...Baker
AVIATION...Meier
FIRE WEATHER...Meier

NWS BOU Office Area Forecast Discussion