Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 260321
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
921 PM MDT Sat Mar 25 2017

.UPDATE...
Issued at 923 PM MDT Sat Mar 25 2017

Updated the weather type grids for Sunday morning through early
afternoon across the Plains HRRR, ESRL HRRR, and NAMnest have all
been trending too warm for snow below about 6500 feet. Adjusted
the weather type grids across all of the plains to be all rain.
The wet bulb temperature is cool enought along the Palmer Divide
above 6500 feet has a rain/snow mix. High res models are trending
drier for the I-25 and metro Denver corridor. PoPs are in good
shape but rainfall amounts will be on the light side (0.10-0.20
range) other than the Palmer Divide area where northerly upslope
will favor slightly higher precipitation amounts. Looks like the
best chance of rain Sunday morning across the Boulder and Denver
metro areas will be from about 8 AM through Noon. The short term
discussion below hits the key points well and the current forecast
follows those details.

Radar and webcams show that snow has started on schedule above
9,500 feet across the Continental Divide and high peaks from
Rocky Mountain national Park south to Summit County. I-70 at and
near the Eisenhower Tunnel already has some snow on the road.
Snow amounts in the mountains above 8,000 feet should stay well
below advisory levels, 2-5 inches across the highest passes are
possible through Sunday evening.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 350 PM MDT Sat Mar 25 2017

An upper level low that just crossed the southern NV/UT
borderlines has spread its cirrus shield over the state. As the
system pushes closer to the forecast area and as surface pressure
falls along the lee of the Rockies, southeasterly surface winds
are increasing. The upper low and surface low will deepen over
southeastern Colorado tomorrow morning, helping to wrap moisture
into northeastern Colorado overnight. A few showers will move into
the mountains this evening with snow levels starting around 9500
feet. Instability and increasing upward QG motion will help a few
rain showers spread over the plains later this evening. The main
cold front should push through the high country this evening then
out onto the plains after midnight. Snow levels are expected to
drop to near 7-7.5k feet with some rain/snow mix down to 6500
before sunrise. As the low spins over southeastern Colorado Sunday
morning, slightly cooler temperatures are expected to be pulled
down to bring snow levels possibly down to 6000 feet or slightly
lower. Overall, not expecting snow for most of the metro areas,
with less than an inch of snow for the Palmer Divide and less for
the Cheyenne Ridge. Some snow over the far eastern plains with
richer moisture and colder temperatures away from any downsloping
effects should result in amounts less than an inch as well.

As the system pulls away Sunday, upward QG motion will push east
as well, for a slow pull away of the precipitation. Northwesterly
flow aloft behind the low will keep light snow over the mountains,
for a total of 2 to 6 inches over the 24 hours. Temperatures
should be about 10 degrees colder. Moderate northerly winds on
the backside of the system will gust in the 20 to 35 mph range
over the plains during the morning and early afternoon.

.LONG TERM...(Sunday night through Saturday)
Issued at 350 PM MDT Sat Mar 25 2017

...Active weather pattern will remain in place this upcoming
week...

Upper level trough will continue to push east and out of the area
Sunday evening. Airmass will stabilize with any lingering showers
ending during the evening.

Attention will then turn toward next upper level storm system
digging into the southwestern U.S. Monday night and Tuesday. The
models seem to have slowed a bit more over the past 24 hours. In
advance of this system, we should see a round of convective
showers develop with daytime heating and sufficient moisture
Monday afternoon and night. A lead short wave kicking out ahead of
the main upper level trough may also help increase shower
coverage. At this time, most of the showers are expected to
develop over the mountains in the afternoon and then spread
northeast across the foothills and nearby adjacent plains through
Monday evening, while the eastern plains may stay dry.

Tuesday and Tuesday night, a closed low is expected to cut off in
eastern Arizona, and then drift slowly eastward into New Mexico
through Wednesday. There is large scale lift ahead of this system,
but given the low is so far south, the forecast area stays in
generally weak but persistent Q-G lift. Precipitable water values
of 0.5 to 0.6 inch across the plains are abnormally high, and
forecast lapse rates are neutral to slightly unstable. Therefore,
any type of forcing whether from Q-G or a light upslope flow
should be enough to generate widespread showers. Will also add
some thunder with forecast CAPE near 500 j/kg Tuesday afternoon in
and near the mountains. The wet weather should continue into
Wednesday before tapering off from northwest to southeast across
the area. Overall, this looks like a moderate precipitation event
shaping up across most of the forecast area. Snow levels are
advertised to be quite high with the very moist airmass in place
and lack of significant cold advection. Expect snow levels to
mostly range between 7500 and 8500 feet through this period,
possibly lowering to 6000-6500 feet by Wednesday morning with some
snow on the Palmer Divide area.

After that storm system exits the area, we`ll have a day or so of
drier weather and near normal temperatures for Thursday. Then the
next storm system may bring unsettled weather back into the
picture by late Friday or Saturday. This storm may end up farther
south but will still have above climatology PoPs and cooler
temperatures in the forecast for this period with a trough around.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 923 PM MDT Sat Mar 25 2017

The three terminals will see lower ceilings and some rain showers
Sunday morning. Moisture has been on the increase in the low and
mid levels of the atmosphere ahead of an approaching storm system
moving across Northeast Arizona into northern New Mexico.
Expecting MVFR conditions at DEN starting around 13Z. Rain showers
look to impact the three terminals between 14 and 18Z. During the
rain showers VIS may be reduced to 3-5 SM and ceilings down to
about 015 kft. Ceilings should improve to VFR all three sites
after about 20Z. Winds tonight and through the morning should
remain around 10-15kts or less, then as the surface low deepends
and moves southeast into the Texas Panhandle, winds at DEN and APA
will come around to north by 13 or 14Z, then strengthen to
15G25kt or so and remain northerly until overnight into Monday.
BJC should remain in the 10kt range or less with a bit more
uncertainty on the wind direction.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Schlatter
SHORT TERM...Kriederman
LONG TERM...Barjenbruch
AVIATION...Schlatter



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