Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 192017

Area Forecast Discussion
National Weather Service Denver/Boulder CO
217 PM MDT Fri May 19 2017

.SHORT TERM...(This evening through Saturday)
Issued at 215 PM MDT Fri May 19 2017

The upper trough anchored to a 500 mb low over south central CO
remains elongated along a north-south axis along the spine of the
Rockies. On GOES-16 one can see a stark contract in cloud
coverage from west-to-east across the state with generally clear
skies west of the Divide and mostly to overcast skies east of the
Divide. Under these clouds, area NWS radars continue to detect
north-south bands of mostly light precip crossing  the eastern
half of the CWA. Only a few showers were still being detected in
the local high country. However, that should change as
temperatures warm with increasing sunshine. On the plains, have
witnessed a slow westward expansion in shower coverage with
transport winds becoming more oriented southeasterly in direction.
HRRR and RAP show this showery precipitation advancing as far
west as the eastern edge of the I-25 urban corridor by 22z today.
More aggressive is the ARW which shows this precip/QPF against
the northern foothills by 21z and and in the Denver metro area
about an hour later. These models also indicate an area of heavier
QPF forming across the northeast corner of the state late this
afternoon and lingering through early this evening. They indicate
up to 0.30-0.40 inch of QPF and 1-2 inches of snow out there.
Elsewhere on the plains, the showery precip this afternoon should
mainly fall as rain, although the heavier showers east of the
urban corridor and up near the Wyoming border may be a mix of rain
and snow. Additional snow accumulation at higher elevations
generally under 3 inches.

Tonight...the 700-500mb low is progged to trek northeast across
sern CO early and nwrn KS late, the mid-level flow over nern CO
become northerly again. This will advect drier air down from WY,
but not necessarily cooler air as temps up there are slightly
warmer than they were down here today. The boundary layer air
filtering down will also be drier, and with an end to the QG lift
and orographic forcing, should see an end to nearly all shower
activity by midnight or so. The question remains, how fast will
low and mid-level clouds clear over the plains. That would mean
the difference between a night of sub- freezing temperatures or
not. At present, a freeze warning is out for the plains which go
into effect at 2 AM MDT. As I see it not expect clouds to
clear sufficiently over the eastern half of the plains zones
before 12Z to allow temperatures to fall below 0C. However, closer
to the foothills, I guess temps could fall to just below freezing
for a long enough time to warrant holding onto the warning.
Therefore, will keep the freeze warning for zones
038..039..040..041..042 and 043 up along the foothills.

On Saturday...As the upper low will continue its northeastward
track across Nebraska, causing the flow aloft to become
northwesterly. This will warm and further dry out the plains
airmass. Latest guidance indicates highs tomorrow in the mid to
upper 50s on the plains, 40s/lower 50s in the high country. That`s
about 10-12 deg f below average for the plains. In the high
country, models show a weak disturbance embedded in this
northwest flow moving down over the nrn mtns during the morning
hours and generating scattered snow showers. Snow amounts should
be light. Expect to see this batch of high country precip winding
down around midday. Then during the afternoon, isolated heat
generated showers forming over the high ridges. High temps in the
high country Friday will be about average for the date.

.LONG TERM...(Saturday night through Friday)
Issued at 215 PM MDT Fri May 19 2017

Zonal flow will prevail over the CWA Saturday night into Sunday
evening. Then an upper trough will push across Colorado on Monday.
Northwesterly flow aloft is progged Monday night. Downward
synoptic scale energy is progged Saturday night into Sunday
evening, then weak upward motion is over the CWA into Monday
evening. Normal diurnal wind patterns appear to dominate the low
level winds Saturday night into Monday. A decent cold front with
upslope behind is progged into and across the plains late Monday.
The upslope continues Monday night. For moisture, there is quite a
bit progged Saturday night well into Sunday night...with some
clearing into midday Monday. Moisture increases Monday afternoon
and night. This is a tiny bit of CAPE over the high mountains
early evening. There is better CAPE around late day Sunday, with
the highest values over the mountains and foothills. There is less
CAPE late day Monday, and only over the mountains. There is a bit
of measurable precipitation over the mountains Saturday evening.
There is more for late day Sunday and it covers most of the of the
forecast area. Same for Monday. Will go with a few alpine showers
Saturday evening. Will need higher pops Sunday and Monday, even
some "likely"s in the high country. For temperatures, Sunday`s
highs are 0-1.5 C warmer than Saturday`s. Monday`s highs are 1-3 C
cooler than Sunday`s. For the later days, Tuesday through Friday,
north-northeasterly winds aloft prevail Tuesday into Wednesday as
an upper ridge moves into the CWA Thursday and Friday. Tuesday
night into Friday morning look to be dry. Temperatures will get
back to seasonal normals from Wednesday onward.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 215 PM MDT Fri May 19 2017

A weak surge of moisture moving westward over the plains is
expected to back into the metro area over the next 1-2 hours.
Could see a few showers form in the area as a result and periodic
ceilings in the 400-700 ft agl range with these showers. Vsbys may
also drop to under 2 miles at times in light precip and fog.
Northerly winds at the moment should turn east-northeast at
8-15kts once this surge arrives. This evening, should see showers
move out of the metro area and the low clouds slowly eroding
eastward. By 05z or 06z should see VFR conditions return to the
metro area. Further clearing is likely late tonight as drier air
aloft migrates down from the north. On Saturday...VFR conditions
will prevail with south-southwest winds of 5-12kts transitioning
clockwise to north-northwest at 8-16kts by the afternoon.


Issued at 215 PM MDT Fri May 19 2017

A flood advisory will remain in effect through at least tomorrow
afternoon for the South Platte River near Kersey in Weld County,
along the South Platte River near Weldona in Morgan County, and
along the South Platte River near Balzac in Logan, Morgan and
Washington Counties.

Runoff from the recent heavy rainfall and the heavy snow cover in
and near the Front Range will continue to make its way down the
South Platte River. This will cause flooding of lying areas along
the river from west central Weld County through Morgan County.
Only light precipitation is anticipated in these areas during the
next 24 hours.


Freeze Warning from 3 AM to 8 AM MDT Saturday for COZ038>043.



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