Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 180929
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
329 AM MDT Tue Sep 18 2018

.SHORT TERM...(Today and tonight)
Issued at 320 AM MDT Tue Sep 18 2018

A few lingering showers continue along our northern border areas
with Wyoming on the heels of 80kt jet streak across Central
Wyoming. Expect these showers to diminish by daybreak as the jet
begins to shift northward away from Colorado. Expect today will be
drier and slightly warmer with less cloud cover, while 700mb
temperatures climb to around +17C. Record high temperature for
Denver today is 93 degrees and forecast should be right around
that record mark for the day. May see a bit of convective
cloudiness late in the day over the mountains and Front Range but
limited moisture and lack of CAPE will keep showers/thunder out of
the forecast. Water vapor imagery doesn`t show any hint of waves
or influx of moisture in this southwest flow over Colorado
through tonight.

.LONG TERM...(Wednesday through Monday)
Issued at 147 AM MDT Tue Sep 18 2018

A robust moisture plume will come up across central and eastern
Colorado on Wednesday ahead of a shortwave that will move across
early Thursday. Meanwhile a cold front will drop into eastern
Colorado ahead of these features. Models suggest quite a bit of
moisture with the plume, but the cooler and only somewhat moist
low level air on the plains will likely restrain the convection.
Despite precipitable water of 1.15 inches on the NAM sounding at
Denver at 00z Thursday, CAPE is quite low. Soundings suggest there
may be up to 800 J/kg near the eastern border, with less as you go
west. This could well start as thermally driven thunderstorms over
the central and eastern part of may have stronger winds along
with low humidities.he mountains, then transition into a big cloud
mass with widespread but weak convection across the plains in the
evening. As a result, we will have fairly high PoPs, especially
on the eastern part of the plains in the evening, but forecast
rather low precipitation amounts. The northwest part of our area,
which needs the rain the most, will likely miss out on all of this
as it will be northwest of the plume. Slim chance of a little
convection on the fringe with some lift from the approaching
shortwave, but it`s not looking promising. Temperatures for
Wednesday are challenging, depending on the timing and strength of
this initial cool surge and the cloud cover. It could stay in the
70s, or get into the mid to upper 80s. The temperature gradient
does appear to be fairly strong, which should help push the cool
air in, so we lowered the highs a bit.

Thursday will see much drier air aloft coming in, with some wind
and subsidence behind the trough. Not a lot of wind, but there is
some concern about the strength of the winds over the mountains.
The models have been consistent with stronger winds aloft early
Thursday, diminishing by Thursday night, but have varied on how
far south the stronger (25 knots at 700 mb) winds come. NAM has
more aggressive about bringing these into northern Colorado,
while the GFS has held them in Wyoming. Latest GFS is trending a
bit stronger into Colorado. In any event, there should be at least
a little increase in the higher mountain winds coupled with
pretty low humidities. If there`s enough sun and mixing,
temperatures could be a little warmer on Thursday despite being in
the cooler air mass on the plains despite another little surge of
northeast winds.

Friday looks to be a fairly quiet day with less flow aloft.
Returning southerly low level flow may bring in enough
warmth/moisture for a few storms on the southern fringe of our
area. More likely it will still be too stable. Saturday looks
similar, though likely with some warming.

There`s reasonable consensus on a stronger trough digging into the
west early next week. This should lead to a little warming and
increased southwest flow ahead of the trough, probably associated
with very dry air at least initially. This should be followed by
some cooling and at least a day of moderate winds aloft behind it.
Timing/strength/latitude and whether there is any moisture with
the trough is uncertain, but it does look like a significant
chance of having 1-3 days of stronger winds (perhaps something
like 30 knots at 700 mb) along with low humidities somewhere in
the Sunday-Tuesday time frame. Model blend is fine for the
temperatures for now--but note that there will likely be stronger
winds with the trough whichever day it comes on.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 320 AM MDT Tue Sep 18 2018

Surface winds have shifted back southwest at KDEN/KAPA, however
weak showers near Wyoming look to be sending another boundary
southward towards the terminal sites.  Winds could return back
northerly between 10-11z. Expect more weak easterly winds this
afternoon like we saw yesterday. A bit drier today with still a
bit of convective cloudiness by late in the day.

&&

.FIRE WEATHER...
Issued at 320 AM MDT Tue Sep 18 2018

Still elevated fire weather conditions again this afternoon with
near red flag conditions over the high mountain valleys of
Jackson and Grand counties with the usual gusty west winds up to
25 mph and humidities between 15-20 percent. Drier today with less
convective cloudiness over northern sections.

There is a chance of wetting rains Wednesday afternoon and evening
from Park county and the Front Range foothills eastward, but areas
further west have only a slight chance of light rain. There should
be a period of stronger winds over the mountains Wednesday night
into Thursday, along with much lower humidities developing by
Thursday afternoon. Winds will primarily be from the west or
northwest during this time. It will be cooler on the plains.

Temperatures will remain cool on the plains Friday, then gradually
warm through Sunday with lowering humidities. It will remain warm and
dry over the mountains. Winds will generally be light, but with
some afternoon breezes.

Another trough will move across Colorado early next week. It is
too early to specify details, or which days will be worst, but this
trough may have stronger winds accompanied by low humidities
somewhere in the Sunday-Tuesday time frame. At this time, the
chance of precipitation with the trough looks low.

&&

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

$$

SHORT TERM...Entrekin
LONG TERM...Gimmestad
AVIATION...Entrekin
FIRE WEATHER...Entrekin/Gimmestad


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