Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 241459
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
859 AM MDT Sun Sep 24 2017

.UPDATE...
Issued at 852 AM MDT Sun Sep 24 2017

Radar and observations showing very light rain/drizzle increasing
over the Front Range urban corridor during the past hour.
Beginning to see enhance lift from next jet core which is now
lifting northeast out of northern New Mexico. Meanwhile the main
upper low begins to move slowly northeast across west central
Utah. Main changes to forecast will be to increase rain chances
this morning while dropping afternoon temperatures into the upper
40s to near 50 with steady temperatures for rest of today.
Mountains remain mostly sunny, except low clouds and patchy fog
over mountain valleys. Showers will increase over the mountains
later this morning and afternoon with destabilization and lift
from the upper low.

&&

.SHORT TERM...(Today and tonight)
Issued at 452 AM MDT Sun Sep 24 2017

Lift associated with the jet streak continues to move slowly east.
Light rain continues over eastern Colorado border areas, with some
drizzle from the boundary layer moisture further west. Near the
mountains there has been some thinning of the low level moisture
as the lift is gone and there was even a very light puff of west
wind a few hours ago which has now died. As a result there is
less drizzle and some holes in the lowest cloud layer over the
Front Range cities at this hour. Meanwhile most of the mountains
are in the dry air aloft behind the trough. Some fog has formed in
North and Middle Parks.

Not much will change with the cool moist low level air over the
plains today. Surface pressure gradient will be weak, but there
will likely be a tendency toward renewed weak upslope flow from
the low pressure center lifting from Utah into Wyoming and diurnal
effects. In addition, what little heating there is at the surface
will probably cause some convective filling in of the cloud deck.
Current forecast has very little temperature rise today. Model
guidance is even a couple degrees colder and should be right if
the clouds remain thick, so I nudged temps down a little more.

As for precipitation, there should be a further decrease this
morning as the jet-related lift moves away. Then we will come
under the influence of the low as it passes. There would probably
be some weak diurnal convection over the mountains anyway, but the
low should provide a little lift and a little destabilization to
help things along. There will also probably be a little lift
working on the boundary layer moisture to produce some shallow
showers and/or drizzle even away from the mountain/low related
convection. I bumped PoPs up a bit more this afternoon and
evening, though amounts will probably not be much outside of the
northern mountains.

Snow levels have been running a bit lower than expected through
this event--before the snow ended early this morning the level was
between 8 and 9 thousand feet. This should be higher this
afternoon and then lower a bit again tonight. Still warm enough
that it shouldn`t have much impact on the highways, though a good
shower could slush up the higher passes.

.LONG TERM...(Monday through Saturday)
Issued at 452 AM MDT Sun Sep 24 2017

On Monday the upper level trough will be over central WY with its
axis stretching to the southwest towards the Four Corners. There
will be a weaker and relatively drier southwesterly flow in the
mid levels. Still enough moisture around however in the lower
levels for some light showery precipitation, generally isolated to
scattered coverage. Monday night and Tuesday, the trough will
lift to the northeast with a short wave ridge and subsidence over
the region. Temperatures will moderate as the airmass dries out
on Tuesday. Another trough will be over the Great Basin and Desert
Southwest Tuesday, this feature becomes a closed low by
Wednesday. Very little flow aloft over the cwa on Wednesday as we
sit in a deformation zone. The surface high will be over the
Midwest by Wednesday with a southeasterly flow increasing in the
afternoon. The best chance of showers/tstms will be over the
mountains, with isolated coverage along the Urban Corridor. Mid
level moisture will increase over western CO for Thursday and
Friday as the closed upper trough lifts northeast. It will be
located along the CO/UT border Thursday night. Confidence on the
track of this feature drops off with differing outcomes generated
by the models by the end of the week. There may also be a weak
front that backs into northeast CO Thursday night, but not much
impact as temperatures over the northeast plains will continue to
warm on Friday. Will continue to keep chc pops in the high country
for Friday and Saturday, but mainly dry over the northeast plains
of CO.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 852 AM MDT Sun Sep 24 2017

Ceilings lowering again as next batch of ascent brings the lighter
rain and drizzle back into the terminals. Have already adjusted
terminals for the lower ceilings and rain this morning.  Expect
these conditions to persist through this evening with no changes
to the later periods.


&&

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

$$

UPDATE...Entrekin
SHORT TERM...Gimmestad
LONG TERM...Cooper
AVIATION...Entrekin



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