Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 210335
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
835 PM MST Fri Jan 20 2017

.UPDATE...
Issued at 808 PM MST Fri Jan 20 2017

Weak short wave ridge axis over central CO with weak mid level qg
descent over much of the cwa this evening. Some moisture
increasing from the southwest but should not impact the north
central mountains of CO until after 12z Saturday. The latest
RAP13/HRRR runs show some showers developing primarily in zone 31
late tonight, but overall not much elsewhere across the cwa. The
main area of showers should still be over west central and
southwestern CO by 12z Saturday. As a result, no significant
updates are planned to the forecast grids at this time.

&&

.SHORT TERM...(This evening through Saturday)
Issued at 350 PM MST Fri Jan 20 2017

Radars shows a few snow showers left over the plains. Models have
an upper ridge over the CWA overnight with weak southwesterly
flow aloft. The flow aloft stays weak on Saturday but is
southwesterly, then westerly by afternoon. The QG Omega fields
have downward motion over the forecast area tonight, then upward
on Saturday. The boundary layer wind and surface pressure fields
keep a weak gradient over the CWA the first two periods along with
normal diurnal wind patterns. Models show a fairly dry night
tonight, then moisture increases from west to east on Saturday. By
afternoon, moisture is pretty deep over the mountains. There is
nothing on the QPF fields tonight, then some moves into the
western CWA by Saturday afternoon. For pops will go with 0-20%s in
the high mountains tonight. Will increase pops through the day
Saturday for the mountains with 40-60%s by afternoon. There should
be some orographic enhancement late of Saturday. Will keep the
plains dry both periods. For temperatures, Saturday`s highs will
be a tad cooler than this afternoon`s highs.

.LONG TERM...(Saturday night through Friday)
Issued at 230 PM MST Fri Jan 20 2017

Sunday is a transition day in between storm systems. The Pacific
storm impacting Colorado on Saturday moves east-southeast into the
Red River Valley of Oklahoma by Sunday morning quickly drying out
the atmosphere over Colorado. At the same time a strong long wave
trough develops off the coast of California, which builds a
ridge that moves from the Great Basin to over Colorado throughout
the day Sunday. Despite the synoptic scale subsidence and
advection of dry air, there could be enough orographic lift for
light snow midday Sunday along the Park Range and other northern
mountains. The rest of the area should see a dry and seasonably
warm day. The ridge axis slides east and Pacific moisture returns
to the mountains Sunday night into very early Monday. A small
amount of synoptic lift from the approaching trough, combined the
approach of a 150 kt southwestern jet streak should be enough to
produce good snow across the mountains Sunday night through
Tuesday. Highlights may be needed for some of the favored west
and southwest upslope areas west of the Divide. Across the Plains
there is enough lift generated despite a lack of easterly upslope
that there is a slight chance of initially rain late Monday
afternoon which would change over to snow by Monday evening.
Amounts look light and confined to areas along and west of I-25.

Monday night into Tuesday the trough axis moves across NE
Colorado and there are chances of light precipitation across the
northeast Plains. Cooler air filters into Colorado from the north
and any precipitation that falls on Tuesday should be snow.
Mountain snow should continue through Tuesday with generally
lighter amounts compared to Monday. Best chances of accumulating
snow on Tuesday in the mountains would be across the northern
mountains north of I-70.

Wednesday Through Friday: The progressive and unsettled pattern
continues across the CONUS. A strongly positive tilted long wave
trough extends from southern California east-northeast to the
upper Mississippi Valley. A closed 500 mb circulation looks to
exist across Iowa and Minnesota Wednesday morning and moves it
into eastern Canada on Thursday. GFS and ECMWF has similar height
fields over Colorado but handle the mid level flow across Colorado
a little differently, with the GFS more northerly at mid and
upper levels and the ECMWF more northwesterly. The model blend
and ensembles have light mountain snow Wednesday morning so will
keep low PoPs there. Overall, moisture is lacking given the mid-
level flow originates from western Canada and the sub-tropical jet
stream looks to be well south of CO across the U.S.-Mexico
border. This is especially true Thursday and Friday where it
should be dry across the CWA. With the northerly flow temperatures
should remain below all the way through Friday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 808 PM MST Fri Jan 20 2017

VFR conditions with no significant impacts to the terminals
through 18z Saturday morning. Typical diurnal winds through
Saturday morning. There could be periods with ILS restrictions
developing after 21Z Saturday afternoon, with bkn cigs 040-050 as
a surface low develops near the OK/TX panhandles, so may address
this in the 06z taf issuance.

&&

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

$$

UPDATE...Cooper
SHORT TERM...RJK
LONG TERM...Schlatter
AVIATION...Cooper



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