Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 130312

Area Forecast Discussion
National Weather Service Denver/Boulder CO
912 PM MDT Mon Mar 12 2018

Issued at 906 PM MDT Mon Mar 12 2018

No major adjustments at this time. A narrow line of light rain
showers extends from around Ft Collins to Last Chance. The HRRR
and RAP show these showers diminishing and shifting eastward this
evening. These trends already reflected in the grids.


.SHORT TERM...(This evening through Tuesday)
Issued at 313 PM MDT Mon Mar 12 2018

Isolated shallow convective showers have been developing this
afternoon across the mountains and Wyoming border area. Those will
likely continue for a few more hours into early evening before the
airmass stabilizes, and the tail end of short wave moving through
Nebraska passes by. On the plains, scattered to broken mid level
clouds abound with some virga, and can`t rule out a sprinkle/
flurry briefly reaching the ground. Otherwise look for clearing
skies later tonight as weak subsidence and drying move in.

On Tuesday, drier northwest flow aloft will prevail as a high
amplitude upper level ridge moves toward the state. This will
lead to more sunshine and dry conditions, and temperatures a
couple degrees warmer than today.

.LONG TERM...(Tuesday night through Monday)
Issued at 313 PM MDT Mon Mar 12 2018

Weak northwest flow continues across Colorado Tuesday night as
another Pacific long wave trough moves onshore by early Wednesday.
Downstream of the trough, we will have a ridge overhead through
Thursday afternoon or so. No chance of precipitation area-wide and
the 700 mb ridge and associated warm air builds significantly
from Wednesday into Thursday. 700 mb temps warm from 0 to -2 degC
on Wednesday to +4 degC on Thursday. This will support high
temperatures in the low 70s both days across the Plains. As early
as Thursday afternoon the global models being to diverge. GFS and
NAM advect colder air from 700-500 mb as the west coast trough
approaches Colorado, dropping the temperatures across the
mountains on Thursday, while the ECMWF keeps the colder air west
of us until late Thursday into Friday. Will drop temps a bit in
the mountains for highs on Thursday but it should still be in the
40s to low 50s there. Main impacts through Thursday afternoon
would be for fire weather across Elbert and Lincoln county where
RH will be more than low enough and winds should come up out of
the southwest in the 20 mph range with higher gusts.

Things get interesting from a Numerical Weather Prediction
standpoint late Thursday. The trough approaches Colorado and
moisture embedded in the flow should kick off orographic snow
showers in the Central and northern Mountains late thursday into
Friday. Models vastly differ on the pattern as the ECMWF has a
fairly deep 700 mb low across eastern Colorado Friday morning and
the GFS has a 700 mb ridge. The GFS hangs the trough back over
the Pacific northwest as another strong Pacific short wave trough
digs southwest. The ECMWF splits off a decent short wave trough
that moves east over the state by Friday morning. Obviously snow
amounts would be greater across the mountains under the ECMWF
solution and there would even be a decent chance of convective
precipitation across our area, though along the I-25 corridor the
low-levels remain quite dry. The best chance of convection with
the ECMWF solution would be the mountains and far northeast
Plains. We can`t bite off on either solution yet because there is
so much disagreement and the bulk of the long wave trough is still
a ways off the Pacific coast. All that said the forecast
confidence starting early Friday is very low and the PoPs area-
wide reflect this: high in the mountains, slight chance on the
plains. If the ECMWF verifies a good batch of wrap-around
precipitation is possible across much of the Plains, including the
I-25 corridor. Will wait for some run-to-run consistency from all
models before leaning either way. In terms of temperatures the
GFS has strong return flow across the Plains while the ECMWF has a
surface cold front and colder temperatures aloft throughout
Friday. With the 700 mb trough, a surface cold front, and low
clouds across E. CO, the ECMWF has highs in the 30s across the
northeast Plains, while the dry and heavily downslope flow with
the GFS has highs in the 60s to low 70s. The Canadian model is
sort of in between the two models, perhaps siding a tad more with
the ECMWF. With neither solution looking more likely than the
other, will split the difference and go with near 60 Denver metro
to Fort Collins and Greeley and near 50 northeast Plains. These
numbers will surely change as new data come in and model agreement
hopefully increases.

Over the weekend the models have a long wave trough across the
Pacific northwest and moist southwest flow increasing across
Colorado and continued low confidence in the forecast. Saturday
should be dry and seasonably warm with highs in the low 60s
possible across the Plains. Late Saturday into Sunday
precipitation chances increase across the mountains, but the ECMWF
takes the best lift to our north, leaving the Plains dry. The GFS
is much wetter, especially later on Sunday with good mountain
snows and banded precipitation across the far northern quarter of
the Plains. Support for the banded precipitation on Sunday from
the GFS is strong frontogenesis around from the surface to 700 mb
and a strong subtropical jet stream just south of Colorado. It
also has a 992 mb low across southeast Colorado Sunday, with a
deformation zone supporting precipitation across northern Colorado
wrapping back into the Denver metro area. GFS temperature
profiles would support rain across most of the Plains, and snow
possible above 6000 feet. The ECMWF lifts the trough north of us,
leaving the Plains dry and seasonably cool this weekend.
Regardless of solution(s) mountain snow with the moist southwest
flow is a safe bet and will retain chance PoPs there starting late
Saturday and continuing into early Monday. The Plains are a blend
of the dry ECMWF and wet GFS, with 10-25 percent PoPs. We`ll be
watching the trends closely given this unusually challenging


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 906 PM MDT Mon Mar 12 2018

VFR conditions will persist through Tuesday. Scattered to broken
mid level cloud deck around 10 thousand feet, briefly lower to
around 7000 feet over the next hour or so, with some virga around
as well through about 05Z. Then look for gradual clearing after
05Z. East/northeast winds up to 10 knots should slowly decrease
03Z-05Z, and then eventually become light and variable late
tonight after 08Z. Light diurnal east/southeast expected to
develop by 18Z-19Z Tuesday.




SHORT TERM...Barjenbruch
LONG TERM...Schlatter
AVIATION...Cooper is the U.S. government's official web portal to all federal, state and local government web resources and services.