Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 251058
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
458 AM MDT Mon Sep 25 2017

.SHORT TERM...(Today and tonight)
Issued at 458 AM MDT Mon Sep 25 2017

Lots of subtle things happening this morning. Moist cool low level
air remains in place, but there was a little drier air working in
from the northern border into Denver since midnight. This brought
a little clearing, which has now largely filled in again after a
couple degrees of cooling. Meanwhile, low levels are still moister
out on the plains and south of Denver. Another jet streak will
pass over eastern Colorado today. This is producing rain over the
southeast border areas, and lighter showers further west as far as
Salida at this time. This will spread north across northeastern
Colorado and should bring an increase in the light rain and/or
drizzle later this morning into the afternoon. This may be moving
away from our area by late afternoon. Meanwhile, there is
agreement on an area of lift rotating out from underneath the low
across the northern mountains in the afternoon and possibly early
evening. Like the wave yesterday, this should bring some showers
to the mountains, and the lift could enhance the drizzle again
late in the day, mainly north of Denver. One difference is that
there is this pocket of low level dry air currently north and west
of Denver. Over the plains this may be wiped out by the midday
wave, but the low level dry air is better established over the
mountains, and should limit the amount of precipitation there
today.

For tonight, drier air aloft moves in, and the low/mid level
drying will also try to edge eastward. However, moisture will
remain south of a mid level deformation zone, across southern
Colorado. Some disagreement on how this boundary shifts. Better
odds of clearing across areas north of Denver, but it could spread
south to Denver and Limon during the night before coming northward
again Tuesday morning. This will have some impact on low
temperatures, though areas that clear will see a lot of
condensation, possibly patchy fog, and possibly frost in the
colder spots. It should be too warm to worry about freezing on the
plains except maybe in the cool spots northeast of Greeley, but we
will have to try to pin down the clearing and frost potential
today.

.LONG TERM...(Tuesday through Sunday)
Issued at 458 AM MDT Mon Sep 25 2017

Tuesday and Tuesday night, the models continue to show a cool but
dry west/southwest flow aloft as the first trough exits to the
north and east and another digs into the Great Basin. Only qpf
generated in the models appears to be around South Park in the
late aftn and early evening period. Wednesday and Wednesday
night, a weak diffluent southerly flow aloft will be in place
with the closed upper low over northern Arizona. Temperatures will
moderate a bit with moisture increasing across southern and
western CO. Will keep chance pops primarily in the mountains with
the highest coverage south of Interstate 70. Aside from isolated
pops near the foothills and Palmer Divide, no qpf generated by the
models over the northeast plains. Thursday into Friday, the
models want to lift the closed low to the north and east. It is
progged to be along the CO/UT line by midday Thursday, then opens
up as it continues to lift to the north and east, over
southeastern WY by 12z Friday. Warming trend will continue, with
enough heating to include a chance of thunderstorms as well.
Highest coverage will be in the mountains, with isolated to
scattered coverage over the northeast plains both days.
Temperatures will be at to slightly above normal by the the
weekend. GFS and ECMWF are about 90 degrees out of phase at that
time, so confidence in the weekend period is on the low side.
Generally slgt chc pops in the grids both days, primarily in the
mountains.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 458 AM MDT Mon Sep 25 2017

MVFR conditions will prevail through this evening, with areas of
IFR until about 16z. There may be some clearing after 06z tonight,
but there also may be some patchy fog developing Tuesday morning
so there is a chance of MVFR or IFR conditions for Tuesday morning one
way or the other.

&&

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

$$

SHORT TERM...Gimmestad
LONG TERM...Cooper
AVIATION...Gimmestad



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