Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 220301
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
901 PM MDT Sat Apr 21 2018

.UPDATE...
Issued at 857 PM MDT Sat Apr 21 2018

Blended in latest temp and wind trends, otherwise forecast in fine
shape. Clouds hanging in over the plains, and as yet little if any
fog formation. Precip has finished up and removed last of the pops
in the mountains with this update. Wind turning to nocturnal
drainage pattern.

UPDATE Issued at 602 PM MDT Sat Apr 21 2018

Quick update to the forecast to blend in current temperatures and
wind. Precipitation all but over, any lingering on the plains will
be done by 7 pm with light snow or rain/snow mix lingering a
little longer in the mountains. Solid cloud cover on the plains as
the low pressure area pulls out, but expect rapid clearing before
midnight with downslope flow. Fog already in the forecast, but
will need to watch closely for dense fog with more reduced
visibility when skies clear and we radiate out.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 305 PM MDT Sat Apr 21 2018

Light snow over the Front Range foothills and northeast Colorado
plains has been gradually diminishing over the past couple hours.
This trend will continue into the evening with some clearing as we
head towards midnight. Light northerly flow across the forecast
area is going to remain fairly moist. Therefore, no drastic
clearing is expected. Cloud cover could diminish after sunset due
to the loss of diurnal heating. But as skies clear, conditions for
strong radiational cooling will develop. This could lead to the
development of areas of fog, especially over the plains. Have
added areas of fog across most of the plains since it is hard to
tell where the most clearing and therefore the best conditions for
radiational fog are going to be. Temperatures this afternoon have
only made it to the upper 30s and lower 40s, so it will not take
much cooling to get down to the dew points in the lower 30s. If
there is a drastic decrease in the cloud cover this evening, then
areas of dense fog may be possible. Will not issue any highlight
for that due to the high uncertainty that all conditions will be
coming together.

For Sunday, Spring will return with skies becoming partly to
mostly sunny and temperatures rebounding into the 60s across the
plains. An upper level ridge will be building over the state, but
short and medium range models indicate the possibility of some
afternoon showers developing over the mountains. Then again,
afternoon showers over the mountains are a Spring-like thing.

.LONG TERM...(Sunday night through Saturday)
Issued at 305 PM MDT Sat Apr 21 2018

Overall there is not much agreement between the ECMWF and GFS for
next week.  The first thing to deal with is an upper level trough
that is supposed to move into the nrn Rockies late Monday into Tue.
The GFS has this feature eventually closing off over the ern
Dakota`s by Tue aftn while the ECMWF has this feature over sern WY
and wrn NE at the same time.  Ahead of this system cross-sections
show an increase in moisture by Mon aftn with favorable lapse rates
over the higher terrain so will see a good chc of showers and a few
tstms.  Over nern CO will see a chc of showers and possibly a tstm
in the aftn as well. Highs will range from the mid 60s to lower
70s across the plains.

For Mon night a cold front will move across nern CO as portion of
the upper level trough mentioned above affects nrn CO.  Thus will
see a better chc of showers and possibly a few storms across the
plains.  In the mtns shower activity may decrease as drier air
spreads across in the evening.

On Tue, a lot depends on where the location of the upper level
low is. Naturally if it ends up over the ern Dakota`s pcpn chances
would be fairly low across the area. However if the upper level
low ends up being over sern WY/wrn NE, or even further south than
shown, then there could end up being a better chc of pcpn. For now
will just keep pops in the 20-30% range. Meanwhile it will be
cooler on Tue as readings over nern CO range from the mid 40`s to
around 50.

By Wed the upper level trough should exit the area with drier
conditions and warmer temperatures.  Moving into Thu, once again
there are major differences between the ECMWF and GFS.  The ECMWF
has another upper level trough in NNW flow aloft moving across the
area along with a decent cold front.  This would lead to another
chance of pcpn along with colder temperatures.  Meanwhile the GFS
has dry NW flow aloft and basically no front which would lead to no
pcpn and little change in temps.  At this time have rather low
confidence in what will occur. Thus will keep pops on the low side and
have slightly cooler temps.

On Fri the flow aloft will be dry NWLY.  The GFS attempts to
increase moisture over swrn areas by aftn while the ECMWF is dry. At
this time will trend towards the drier ECMWF and will not mention
any pcpn. Temperatures should be a few degrees above normals.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 602 PM MDT Sat Apr 21 2018

Conditions improved to VFR at 00z issuance time, however
forecasting IFR vis and cigs after 05z as fog develops. Light
drainage wind, generally south to southwest remaining less than 10
kts. After fog/low stratus burn off by 14z vfr with sct clouds,
and surface winds coming around through northeast to east by end
of TAF period.


&&

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

$$

UPDATE...Hanson
SHORT TERM...Dankers
LONG TERM...RPK
AVIATION...Hanson



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