Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 180402

Area Forecast Discussion
National Weather Service Denver/Boulder CO
902 PM MST Wed Jan 17 2018

Issued at 902 PM MST Wed Jan 17 2018

Just a few high thin clouds drifting in from the northwest this
evening. Good radiational cooling setting up for valley locations
again with light winds and dry atmosphere. GOES-16 3.9 micron
imagery showing warm downslope in place across the foothills, and
light downslope winds there should keep them on the warmer side
overnight. Forecast on track.


.SHORT TERM...(This evening through Thursday)
Issued at 355 PM MST Wed Jan 17 2018

Dry and warmer conditions are expected through Thursday under an
upper level ridge. Warming will be due, in part, to 700mb
temperatures rising to near 3 degrees C. Latest satellite imagery
shows cirrus over Montana. Some of this moisture should spill
over the ridge and move into Colorado tonight potentially leading
to thin clouds. Impacts from the thin clouds on temperatures
should be minimal.

.LONG TERM...(Thursday night through Wednesday)
Issued at 355 PM MST Wed Jan 17 2018

Thursday night, a positively tilted upper ridge passes over the
forecast area causing the northwest 700-500 mb flow to turn
southwesterly overnight. The shift in direction begins ushering in
high-level moisture as well as warmer air with a 4-5 deg C increase
in the 700-500mb temp from 12z/Thu to 00z/Fri. Model guidance
indication max temps on the plains in the mid 60s to low 70s in the
greater Denver metro area with the help of a swly/downslope wind.
will be most pronounced. Farther out across the plains, temperatures
not quite as warm as per the models. Se little reason to go against
them, although not all that sure we`l see any 70 deg readings. That
said, the record high for Denver on Thursday is 68F. I feel pretty
confident we`ll at least tie this record, assuming the high-level
mtn wave cloud band doesn`t become as thick as that indicated on the
GFS. High country temperatures also quite warm for this time of year
with 40s and lower 50s on valley floors. If it were not for 25-35
pct min RHs, the 10-20kt swly winds in North and South Parks Friday
afternoon would further elevate wild fire concerns in these areas.
Min RHs near 15 pct along the foothills and Palmer Divide adjacent
to the Denver area Fri afternoon, but wind speeds are generally
expected to be less than 15 mph. While winds will be much stronger
on the high mtn ridges where gusts will be in the 35-50kt range.

Friday night into Saturday...mid and high-level moisture streaming
up from the Desert SW ahead of an open wave upper trough will spread
over the region during the period. Model consensus has this trough
moving over Nevada Friday night and over Utah on Saturday. The mean
layer flow just ahead of the trough is nearly longitudinal which
will continue to advect warm mid-level air over eastern Colorado
during the day Saturday. And, the low-level flow on the plains is
still south-southwesterly. So, one more day of above average
temperatures, warmest along the srn foothills and Palmer Divide
where mid-upper 50s appear reasonable even with increasing and
lowering cloud cover. Model QG fields indicate a steady increase in
large scale ascent through the day and with it the development of
scattered rain and snow showers across the high country. Snow level
starts out high, around 10,000 feet around midday. By evening snow
levels fall to around 6500 ft ASL, with rain showers on the plains
mixing with and changing to snow with the arrival of colder air
behind a not so impressive looking cold front. The GFS drops it
through the Denver metro by mid-evening, while other models hold off
until after midnight. This storm system looks moisture starved based
on mean layer specific humidity and precipitable water fields.
There`s quite a bit of uncertainty as to how much if any snow
accumulation areas east of the Front Range will see by midday
Sunday. Depending on how soon the rain-snow change over occurs,
areas along the Front Range and over the Palmer Divide could see
anywhere from 1/2 to 4 inches, maybe a bit more on the Palmer Dvd
with the NLY post-frontal winds. Do not see all that much upslope
forcing along the foothills, so foothill areas may not receive all
that much snow either. The odd man out, the impressive QG forcing
with this system. Models indicate the strongest/deepest ascent over
the fcst area just after frontal passage. Unfortunately, moisture
isn`t look all that great.

On Sunday, moisture and cold air wrapping around the back side of
this trough may give us our best chance for snow accumulation east
of the mtns. However, by the afternoon with sfc-700mb winds turning
NWLY, could see a fairly dramatic drop off in precip over the along
the Front Range, while mostly light snowfall continues farther out
on the plains. Temperatures not as cold with this system, so high s
on the plains Sunday are expect to be above freezing. Should see a
steady drop off in snowfall in the high country and northeast
plains during the evening hours.

Looks ahead to early next week, dry and slightly warmer conditions
return to the region, although some of the models show a weak
clipper system possibly brushing the higher mtns with snow and
gusty winds on Tuesday.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 902 PM MST Wed Jan 17 2018

VFR conditions with only increasing high clouds above 20 thousand
feet AGL through Thursday night. Normal diurnal wind patterns
will occur through this period.




SHORT TERM...Sullivan
AVIATION...Barjenbruch is the U.S. government's official web portal to all federal, state and local government web resources and services.