Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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241
FXUS65 KBOU 230145
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
645 PM MST Thu Feb 22 2018

.UPDATE...
Issued at 640 PM MST Thu Feb 22 2018

Main convective snow band now hangs over the foothills in northern
Jefferson and southern Boulder Counties, and northern portions of
the Denver/Boulder metro area. A bit ago spotters under the core
of the snow band in the Wheatridge and Arvada recorded 2-3 inches
of snowfall in a short period of time. This core has since moved
very slowly east and was now over the Northglenn, north Denver and
Thornton areas. Those areas could receive a quick 1-2 inches of
snow before this narrow band of light to moderate snowfall moves
east of the area. HiRes models show this and other weak snow bands
moving off the foothills weakening in the next 30-90 minutes and
eventually dissipating farther out on the plains before midnight.
GOES-16 Water vapor satellite imagery shows a weak mid- level
disturbance moving eastward over the I-25 corridor at the moment.
The back edge of this disturbance, ie., marked by a drier air, is
now passing over the Continental Divide. But, this break in the
moisture appears small, with yet another batch of mainly mid-level
moisture expanding over western Colorado. Model Q-G omega fields
indicate a widening area of mostly weak large scale ascent
associated with this west slope moisture. This region of moisture
and lift is projected to reach the Front Range and adjacent plains
of northeast Colorado by Friday morning. We`ll leave that for
later. For now, have updated the forecast grids for the next 6
hours to raise PoPs on the plains, esply over northern portions of
the metro area and to add snow accumulations as described above.
At this late hour, still on the fence as to when and if fog will
back into the I-25 corridor. To play it safe added areas of fog
late tonight for areas north through east of the Denver.

&&

.SHORT TERM...(This evening through Friday)
Issued at 157 PM MST Thu Feb 22 2018

Radars continue to show widely scattered snow showers moving
northeastward across the CWA this afternoon. They will not cause
much accumulation. There is a better looking batch of cloudiness
and radar returns just upstream. This should impact the
northwestern half of the CWA this evening. Strong southwesterly
flow aloft is expected to continue tonight over the CWA. An upper
trough over the Great Basin will move slowly eastward tonight and
Friday. The trough axis will be over central Utah by 00Z late
Friday. There is decent synoptic scale energy on Friday afternoon
for all of the CWA. The boundary layer flow continues to be pretty
weak overnight. On Friday, decent southeasterly flow is expected
late morning through afternoon. Looking at moisture, there is
quite a bit this evening, then it decreases after 06Z tonight
through 18Z on Friday. Moisture increases significantly Friday
afternoon. Cross sections keep moisture around most of tonight in
the lowest level over the plains so will keep some mention of fog
as the current grids have. For pops tonight, will go with
"likely"s in the high mountains, then decreasing after midnight
through noon Friday. Will go with "chance"s for the western half
of the plains this evening, then nothing overnight into mid Friday
afternoon for all the plains. On Saturday afternoon, will
increase pops again for much of the CWA. For temperatures,
Friday`s highs will be 1-4 C warmer than this afternoon`s highs.

.LONG TERM...(Friday night through Thursday)
Issued at 157 PM MST Thu Feb 22 2018

The upper trough over Utah Friday evening will swing east through
the night, increasing QG lift over northeastern Colorado.
Expecting some convective snow lining up in southwest to northeast
bands across the mountains, likely heaviest during the evening
hours. Across the northeastern plains, models have been handling
the system differently. GFS shows slightly more digging by the
upper trough along with much deeper moisture resulting in higher
upward QG values and higher QPF values, especially over the far
northeastern plains where up to 10 inches of snow may occur. NAM,
GEM and to a lesser extent the ECMWF show a much shallower
moisture layer, shallow enough to present the threat of freezing
drizzle along with some snow. However, so much QG lift may inhibit
much of the freezing drizzle potential, however hard to go against
the steady mention of freezing drizzle in the models. SREF plumes
have been steadily increasing their snow amounts for the past 4
runs, increasing my confidence of an impactful weather event. Will
keep the mention of freezing drizzle along with a change over to
snow with a possible 2 to 6 inches of snow Friday night, mainly
over Logan, Phillips, Sedgwick and Washington counties. Will issue
a Winter Storm Watch for the wintry mix of freezing drizzle and
accumulating snow. For the mountains, another 2 to 4 inches to
fall Friday night before diminishing Saturday morning. Westerly
flow will tend to downslope the urban corridor, though the
convective nature and possible banding may still allow 1 to 2
inches to fall. There may be some freezing drizzle pushing back
west into Weld County at least from the northeastern plains as
moisture depth decreases through the night.

As the trough passes overhead early Saturday morning, strong
subsidence will decrease snow west to east as well as increase
northwesterly winds. This may cause blowing snow over the high
country, and possibly over the far northeastern plains, depending
on how much falls. Gusts of 30 to 40 mph will be possible. Another
weak shortwave will push more snow into the mountains Saturday
evening into Sunday morning, with another 2 to 4 inches expected.
The plains should remain dry with slightly warmer temperatures
Sunday and again on Monday as shortwave ridging moves overhead.

On Tuesday, some moisture begins to push in as another upper
trough drops into the Great Basin and desert southwest. Also, a
backdoor cold front may either keep temperatures the same or lower
them Tuesday even though southwest flow aloft continues. Snow
should push into the high country that evening should stick to
the higher terrain Wednesday as the plains are mainly downsloped,
however the northern plains may see some light accumulations due
to a stronger cold push up there. GFS is much weaker than the EC
or GEM with this trough, so may need to hold on to PoPs longer
depending on model consistency. Otherwise Thursday should be dry
and slightly warmer as the trough pushes east and upper ridging
replaces it.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 640 PM MST Thu Feb 22 2018

Light to moderate snow showers slowly tracking northeast over
portions of the Denver metro area at this hour now impacting
mainly KBJC and KDEN. VSBY at KBJC presently down to a mile, and
2.5 miles at KDEN. Could still see lower VSBYs and both north
metro airports during the next 30-60 minutes (e.g. about 03Z),
with upwards of an inch of snow accumulation possible. Otherwise,
MVFR CIGs will prevail this evening, with local CIGs down to
500-800 feet AGL with passing snow showers. By late this evening,
after 04z, much of this shower activity should east of the metro
area, although areas of very light snowfall or snow flurries may
linger until midnight LST. After midnight, airports could see
partial clearing which could result in the formation of fog. To
what extent, remains uncertain. Will continue to closely monitor
and will add to terminal forecasts as soon as possible if
necessary.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Friday afternoon through Saturday
morning for COZ048>051.

&&

$$

UPDATE...Baker
SHORT TERM...RJK
LONG TERM...Kriederman
AVIATION...Baker



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