Weather Saturday AM – Jan 4, 2013 –

http://www.hpc.ncep.noaa.gov/html/sfcloop/ussatsfc_loopb.html

This image displays watches, warnings, statements and advisories issued by the National Weather Service

 

http://www.hpc.ncep.noaa.gov/html/sfcloop/radsfcus_exp_test.html

The East Coast has had it’s fun with heavy snow and windy conditions and now a short-lived High Pressure Area but that same time we have colder air pushing southward,  but that will be short-lived.   Here in the Raleigh and Greensboro the weather will be cold but relatively dry.   As the High moves off winds will switch around to the South and Southwest,    bringing warmer and moister air to the Greensboro area

Our current temperature    16,   Winds – CALM and RH is already 90%.    It appears as the winds have switched now.    Coldest night of the season so far.      We are subject Mid-latitude Lows called Cyclones by some.   Those systems are what motivates the airmasses to act out.   And if there is moisture you have the makings of a problem.    The extent of the problems in winter depend a few different factors as even terrestrial (hills, valleys,  water source)  all coalesce to bring us snow, freezing rain or even rain and drizzle.

The low clouds you see on a given day can be very near to the ground.    We have a term called ‘Woxof’.    This means the ceiling and visibility and are zero,   meaning it is hard to see.  As the airmass becomes more stable,  the clouds lower.   This cloud type is more typical with high humidity as the airmass is pretty much saturated.

All those highs and lows on the current surface chart indicate a very complex and stormy weather pattern until that leaves.   Generally the frontal boundaries are multi-layered meaning the skies would generally clear faster at the surface and is angled back to the High Side of the front.

…MID MISSISSIPPI VALLEY/OHIO VALLEY/GREAT
LAKES/MID-ATLANTIC/NORTHEAST…

A MAJOR PRECIPITATION EVENT IS EXPECTED WITH A SURFACE LOW
SPINNING UP ALONG THE LEADING EDGE OF A EXTREMELY COLD ARCTIC
OUTBREAK PLUNGING THROUGH THE CENTRAL AND EASTERN U.S. THIS
WEEKEND.  FOR DETAILS ON THE WINTER WEATHER IMPACTS…REFER TO THE
HEAVY SNOW DISCUSSION/QPFHSD…FOR DETAILS ON THE EXTREME
COLD…REFER TO THE EXTENDED FORECAST DISCUSSION/PMDEPD…AND FOR
DETAILS ON THE MASS FIELD DIFFERENCES WITH THIS FEATURE…REFER TO
THE MODEL DIAGNOSTIC DISCUSSION/PMDHMD.

INITIALLY…LIGHT PRECIPITATION IS EXPECTED WITH A STRONG COLD
FRONT PLUNGING THROUGH THE CENTRAL U.S. THIS WEEKEND…ON THE
LEADING EDGE OF AN IMPRESSIVE ARCTIC OUTBREAK.  BY SUNDAY…A
VIGOROUS PIECE OF SHORTWAVE ENERGY LIFTING OUT OF THE BASE OF THE
DEEP VORTEX CARVING OUT OVER THE CENTRAL U.S. WILL QUICKLY DEEPEN
A SURFACE LOW WHILE IT TRACKS OUT OF ARKANSAS…THROUGH THE LOWER
GREAT LAKES…AND INTO EASTERN CANADA.  THE COMBINATION OF GULF
MOISTURE…AND EVEN MORE IMPRESSIVE FAVORABLE JET DYNAMICS…WILL
PRODUCE A WIDE SWATH OF HEAVY PRECIPITATION TO THE NORTHWEST OF
THE LOW TRACK.

AHEAD OF THE SYSTEM…MOISTURE GETTING LIFTED INTO THE WARM SECTOR
SHOULD FUEL INCREASING AMOUNTS OF PRECIPITATION ACROSS THE
MID-ATLANTIC…AND EVENTUALLY NORTHEAST.  BY MONDAY…A
STRENGTHENING WARM FRONT LIFTING OUT OF THE MID-ATLANTIC REGION
SHOULD BRING SOME HEAVIER AND MORE
ORGANIZED…OVERRUNNING…PRECIPITATION INTO NEW ENGLAND.  BEHIND
THE SYSTEM…LAKE ENHANCED SHOWERS ARE EXPECTED DOWNWIND OF THE
GREAT LAKES.

MODELS ARE STILL SHOWING SLIGHT VARIATIONS WITH EXACT SURFACE LOW
TRACK/STRENGTH …WHICH WILL HAVE A LARGE IMPACT ON
LOCATION/AMOUNTS OF THE HEAVIEST QPF AXIS.  FOR NOW…THE WPC
FORECAST RELIED ON THE REASONABLY GOOD AGREEMENT BETWEEN THE 12Z
ECMWF/GFS/CMC.

The above is the forecast reasoning (why they expect snow for example or rain).

latest goes east hurricane sector water vapor image

Satellite photos are depictive of clouds types and naturally high overcasts make it difficult to see lower clouds and we can overlay radar images to see just what the clouds mean.    I will explain some other tools we use to determine the specific forecast called the Terminal Area Forecast (TAF).   Snow is probably harder to forecast than even Hurricanes and perhaps Tornadoes.  To be exact where it is going to snow or rain and the boundary of such areas.

Forecast of Fronts/Pressure and Weather valid Sun 00Z

Timing is everything and the big issue for the Eastern US is when does the High get off the coast and the next front get in.   All along the frontal boundary and ahead of these boundaries are where the weather occur.    This is a weekend forecast.

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