Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 182104

Area Forecast Discussion
National Weather Service Birmingham AL
304 PM CST Sun Nov 18 2018

Tonight through Monday.

--Areas of fog tonight followed by increasing cloudiness; band of
showers to move through Monday into Monday night--

For tonight, conditions look to favor another installment of patchy
fog, potentially becoming dense at rivers and valleys. Lows will
average in the low-mid 40s.

Through Monday and Monday night, the approach and arrival of a
surface front and mid-level impulse will result in an increase in
cloudiness and attendant rain showers. The KBMX 18/12Z sounding
showed a PWAT of 0.40 inches, which is expected to increase to
around 1 inch with this system. Forecast soundings suggest it will
take a bit of time for profile and sub-cloud layer/top-down
moistening; so, some of the initial returns on RADAR may be
associated with virga, but rain showers are expected to occur as the
front moves through. WPC guidance rain totals are light/less than a
quarter of an inch. The front will have cleared our forecast area by
very early Tuesday, taking us back toward a period of fair weather,
outlined below.



Tuesday through Thursday.

In wake of the frontal passage, afternoon highs will likely be
5-10 degrees cooler on Tuesday with mid 50s/60s overall. Clouds
will be on the decrease from northwest to southeast.

Thereafter, the next surface high pressure system and stable
continental airmass envelopes the southeastern CONUS with
west/northwest flow aloft. This will stick around during midweek and
should provide nice weather for the Thanksgiving holiday across
central Alabama. Expecting highs in the 50s/60s overall under mostly
sunny skies. Lows are forecast to be the coldest Wednesday morning
with some areas across the north dropping below freezing, but
improves the next couple of nights as the airmass modifies with
time. This high pressure system is expected to `merge` with a
stronger high centered near the Great Lakes region on Thursday as a
more anomalous upper-level trough exits the Northeast CONUS during
this time. This will transition winds at the surface from
northerly to an easterly component, a possible signal of changing
weather upstream.

Friday through Sunday.

There`s been increasing confidence in a system moving across the
country as we head towards the weekend. Not a lot of forecast
specifics are available at this point as best available guidance
prognostics are still lacking temporal consistency with the speed
of the Rossby Wave. Nonetheless, expectations involve a neutral tilt
trough moving through the Polar jet and interacting with the
subtropical jet near the Mississippi Valley Friday evening. Upper-
level diffluent flow and increasing H5 winds indicate support for
synoptic ascent, along with other ageostrophic lift and associated
cyclogenesis to our west. For now, have placed 50-60% PoPs for
Friday/Saturday across the area. Currently the mid/upper-level
dynamics responsible for this activity is residing near the Gulf of
Alaska, so there`s obvious reasoning with respect to uncertainty at
this time, not to mention the complexities involved with this
persisting split-flow jet stream configuration. Will continue to
watch trends in the future and apply changes to the forecast as



18Z TAF Discussion.

VFR conditions will continue into the evening with mostly clear
skies and a few high clouds over our northern sites. Once again,
overnight fog is expected to bring most sites to IFR or MVFR
conditions as early as 04Z through sunrise with VFR conditions
returning by 15Z. Cigs expected to begin lowering around 18Z as
overcast skies move in ahead of an approaching weak cold front.




Drier conditions will persist today, though there`s a chance for
light rain to move in on Monday afternoon and evening. Still
expecting relative humidity values each afternoon to remain above
critical thresholds. Lake and river valley fog has been observed
the past two nights with better coverage overnight through 8 AM
this morning within saturated near-surface conditions. A Dense
Fog Advisory is en effect through 8 AM this morning. There are no
fire weather concerns at this time.


Gadsden     40  64  43  55  34 /   0  40  50  10   0
Anniston    42  67  44  57  35 /   0  30  40  10   0
Birmingham  45  65  44  56  36 /   0  40  50  10   0
Tuscaloosa  44  65  43  58  36 /   0  40  50  10   0
Calera      43  65  43  57  36 /  10  30  50  10   0
Auburn      44  66  49  59  39 /   0  10  30  10   0
Montgomery  43  68  50  61  39 /   0  10  30  10   0
Troy        44  69  51  62  40 /   0   0  30  10   0




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