textproduct: Raleigh

WHAT HAS CHANGED

As of 210 AM Tuesday...

* Confidence in a significant weekend winter storm, and very cold weather into next week, continues to increase.

KEY MESSAGES

As of 210 AM Tuesday...

1) Very cold temperatures are expected today and tonight. Some moderation in temperatures expected Wednesday, but especially Thursday.

2) A major winter storm is possible this weekend.

3) Extremely cold temperatures are likely Saturday through Monday.

DISCUSSION

As of 210 AM Tuesday...

KEY MESSAGE 1... Very cold temperatures are expected through tonight with moderation mid-week.

Cold high pressure over the Central Plains will settle into the TN valley today, then be centered over central NC tonight into early Wednesday. Very cold temperatures will be felt with highs today in the upper 30s to mid 40s N to S. Lows tonight will drop into the teens to lower 20s.

Temperatures will start to moderate Wednesday when highs reach 45- 50. This moderating trend will continue and peak on Thursday with a breezy SW wind aiding the warm-up. Lows Wednesday night will be in the 30s with highs well into the 50s to near 60 expected Thursday.

KEY MESSAGE 2... A major winter storm is possible this weekend.

Synoptic overview: A very strong Arctic cold front expected to move through the CWA late Fri or Fri evening will introduce very cold air into the region starting Fri night and continuing through Mon, as the high center tracks from the Upper Midwest across the S Great Lakes and Ohio Valley into the Northeast and St Lawrence Valley through Sun (see cold details below). A corresponding 850 mb cold front will also move S through the area Fri night then stall and hold over SC and SE NC through Sat, potentially then lifting NW back into central NC as low-mid level waves track along it. A weak surface low is expected to develop over the central Gulf coast late Sat, but the main player should be a second low that forms just off Savannah GA late Sat and deepens as it tracks NE along the Carolina coast Sat night through Sun.

Models: The latest deterministic models and ensemble members overwhelmingly show a period of some sort of wintry precip over all of central NC this weekend, with accumulations peaking Sat aftn through Sun morning. But differences do remain regarding timing and p-types, and the latter prevents us from providing a lot of specifics at this time as to the kind(s) of wintry precip that will dominate. Wintry weather in central NC essentially requires three things: sufficient moisture, sufficient lift, and sufficient cold air, both at the surface and (for snow) aloft as well. In short, the chances that we see all three factors here is growing. Model- projected incoming PWs of 150%-225% of normal seem quite plausible given the strong influx of both Pacific and Gulf moisture. Strong forcing for ascent appears likely at multiple levels, including mid level DPVA as a SoCal low merges with a polar stream trough over the S-central CONUS (although differences here do influence timing of this system's impacts), upper jet-induced divergence aloft, and intense low level isentropic upglide. Finally, confidence is fairly high that we'll have plenty of low level sub-freezing air with a steady feed of lower dewpoints from the north to help counteract the latent heat release from the freezing process. It's difficult to say at this time if we'll have a pure Miller A or B cyclogenesis pattern, but it does look like the coastal low could deepen quickly and become the primary low. The key could be how far N any warm air aloft can get, as that will influence p-types, and the cyclogenesis pattern will help drive the width and location of corridors of predominant p-type. The NBM mean trend in its last several iterations has been for steadily greater snowfall accumulation over the entire CWA, with the highest values still over the N half. Whatever does fall and accrue (whether freezing precip or snow) could be significant and may have lingering effects well into the next work week. We will continue to closely monitor this potential event. -GIH

KEY MESSAGE 3... Extremely cold temperatures are likely Saturday through Monday.

A frigid Arctic high with a likely 1045-1050 mb central pressure will pour into the Midwest and Great Lakes behind the front. With low level thicknesses expected to drop to 1250 mb or possibly lower and with the air mass sourced straight out of the Arctic, the overnight lows (especially Sat night through Mon night) may approach or exceed daily records, and daily highs may come close to record low daily maxima as well, especially if we have a snowpack. A Cold Weather Advisory or Extreme Cold Watch or Warning may be needed. See climate section below. -GIH

AVIATION /06Z TUESDAY THROUGH FRIDAY/

As of 100 AM Tuesday...

VFR conditions are expected over the 24 hr TAF period. A dry cold front push through central NC tonight which could produce a few hours of 10 knot WNW gusts. Any gustiness should subside by sunrise. Otherwise clear skies are expected.

Outlook: VFR should prevail most of Wed, Thu, and Fri, though can't not rule out a stray shower in the Triad early Thu. A potential winter storm could bring a wintry mix and adverse aviation conditions Sat into Sun. Confidence on details are low but increasing.

CLIMATE

Record Low Temperatures:

January 26: KRDU: 10/1940 KFAY: 14/1963

RAH WATCHES/WARNINGS/ADVISORIES

None.