Haz. wx. outlook 332pm: ...WINTER WEATHER ADVISORY IN EFFECT UNTIL 6 AM EST THURSDAY... THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN MAINE AND NORTHERN NEW HAMPSHIRE. .DAY ONE...THIS AFTERNOON AND TONIGHT PLEASE LISTEN TO NOAA WEATHER RADIO OR GO TO WEATHER.GOV ON THE INTERNET FOR MORE INFORMATION ABOUT THE FOLLOWING HAZARDS. WINTER WEATHER ADVISORY. .DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY SEVERAL INCHES OF SNOW IS POSSIBLE SATURDAY AFTERNOON INTO SUNDAY MORNING. .SPOTTER INFORMATION STATEMENT... SNOW SPOTTERS ARE ENCOURAGED TO CALL OR RADIO IN THEIR REPORTS. ___________________________________________________________________________________________ Forecast discussion 342pm: .SHORT TERM /8 PM THIS EVENING THROUGH 6 PM THURSDAY/... -- Changed Discussion -- STEADIER PRECIPITATION ARRIVES FROM WEST TO EAST BEFORE MIDNIGHT IN ADVANCE OF THE SECOND SURFACE LOW. THE COLDER AIR FILTERING SOUTH AND INCREASING PRECIPITATION INTENSITY SHOULD GRADUALLY WIPE OUT MUCH OF THE WARM AIR THAT REMAINS IN THE COLUMN WITH THE HEAVIEST SNOW LIKELY TO SET UP FROM CENTRAL NEW HAMPSHIRE INTO SOUTHWEST MAINE. LESS QPF IS EXPECTED FURTHER NORTH AND ONLY ADVISORY LEVEL SNOW IS EXPECTED THERE. CLOSER TO THE MASS/NEW HAMPSHIRE BORDER WE`LL SEE MORE MIX HELPING TO HOLD SNOW TOTALS DOWN A BIT. IN BETWEEN...WE SHOULD SEE THE SNOW/SLEET JACKPOT WHERE WE MAKE THE MOST OF BOTH THE AVAILABLE QPF AND COLD AIR. THE WARNINGS AND ADVISORIES REMAIN IN PLACE WITH THE ONLY CHANGE THE ADDITION OF SOUTHERN OXFORD AND CUMBERLAND COUNTIES TO THE WARNING AREA AS THIS AREA SHOULD RESIDE IN THE BAND OF HEAVIEST FROZEN PRECIPITATION. SNOW TAPERS TO SNOW SHOWERS BY MORNING AS THE SURFACE LOW EXITS THE COAST AND WE`RE LEFT UNDER THE INFLUENCE OF THE UPPER LOW. .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... -- Changed Discussion -- LOW LEVEL MOISTURE REMAINS IN PLACE THURSDAY NIGHT AND FRIDAY MORNING AS WEAK WARM AIR ADVECTION TRAPS IN THE MOISTURE UNDER THE INVERSION. THE WEAK WARM AIR ADVECTION COULD RESULT IN LIGHT SNOW (AND PERHAPS EVEN FREEZING DRIZZLE...WHICH WAS NOT INCLUDED IN THE FORECAST AT THIS TIME). WITH THE LOW CLOUDS AND LIGHT PRECIPITATION...GENERALLY WENT ABOVE A MOS CONSENSUS FOR LOWS THURSDAY NIGHT AND BELOW MOS FOR HIGHS FRIDAY. DRIER AIR FOR FRIDAY AFTERNOON AND FRIDAY EVENING SHOULD ALLOW SOME CLEARING...AND COULD RESULT IN COLDER TEMPERATURES FRIDAY NIGHT THAN CURRENTLY FORECAST. THIS COULD ALSO BE INFLUENCED BY THE AMOUNT OF SNOW RECEIVED TONIGHT AND THURSDAY MORNING. WHILE SYNOPTIC SCALE MODELS ARE IN DECENT AGREEMENT FOR SATURDAY AFTERNOON INTO SUNDAY EVENING...THE ULTIMATE DISPOSITION OF THE SECONDARY LOW POSITION WILL HAVE AN IMPACT ON PRECIPITATION TYPE... ESPECIALLY NEAR THE COAST SATURDAY NIGHT. AT THIS POINT...FAVORED THE 1200 UTC GFS FORECAST...AND FOLLOWED IT FOR THE DETAILS OF THE FORECAST. AT THIS POINT...QPF VALUES AND CRITICAL THICKNESSES SUGGEST MAINLY SNOW IN THE MOUNTAINS AND FOOTHILLS...WITH A MIX OF PRECIPITATION TYPES ELSEWHERE. BASED ON THE FORECAST LOW LEVEL FLOW...THERE MAY BE BOUNDARY LAYER ISSUES NEAR THE COAST ON THE FRONT SIDE OF THE SYSTEM SATURDAY AFTERNOON AND SATURDAY EVENING. THIS IS OF COURSE BASED ON SECONDARY DEVELOPMENT IN THE GULF OF MAINE SATURDAY EVENING. THERE IS STILL ENOUGH DISAGREEMENT AMONG MODELS TO CREATE LESS THAN AVERAGE FORECAST CONFIDENCE FOR THIS PERIOD. THE 1200 UTC ECMWF IS SLOWER WITH DEVELOPMENT AND SUGGESTS THAT THE DEVELOPMENT WOULD BE LATER SATURDAY NIGHT AND SUNDAY...AND POSSIBLY TO THE EAST. AT THIS POINT...IT APPEARS AS THOUGH MOST AREAS AWAY FROM THE COAST COULD SEE SEVERAL INCHES OF SNOW...BUT THE DETAILS OF THIS SYSTEM STILL NEED TO BE HASHED OUT. AFTER THE SYSTEM DEPARTS SUNDAY...COLDER AIR POURS INTO THE REGION FOR THE EARLY PART OF NEXT WEEK. NEXT CHANCE OF PRECIPITATION WOULD BE THE MIDDLE OF NEXT WEEK...BUT THERE IS ENOUGH MODEL TIMING AND PHASE DIFFERENCE TO LEAVE THE MENTION OF SNOW IN THE FORECAST WITHOUT MUCH ELSE IN THE WAY OF DETAIL. -- End Changed Discussion -- && Topic tonight: Temperature inversions and warm air over-running cold air scenarios. ( NWS-Salt lake City web page ) What are temperature inversions? On most days, the temperature of air in the atmosphere is cooler the higher up in altitude you go. This is because most of the suns energy is converted to sensible heat at the ground, which in turn warms the air at the surface. The warm air rises in the atmosphere, where it expands and cools. Sometimes, however, the temperature of air actually increases with height. The situation of having warm air on top of cooler air is referred to as a temperature inversion, because the temperature profile of the atmosphere is "inverted" from its usual state. There are two types of temperature inversions: surface inversions that occur near the Earth's surface, and aloft inversions that occur above the ground. Surface inversions are the most important in the study of air quality. ( weather.com ) Overrunning Because the United States is located in the middle latitudes of the Northern Hemisphere, an ample amount of both cold polar air and warm tropical air is available in the troposphere during the winter months. Overrunning occurs when moist, warmer air is directed up and over a mass of colder air at the surface of the earth. The warm air cools as it rises, and its moisture condenses into precipitation-producing clouds. This type of weather pattern typically occurs ahead of a trough in the upper level of the atmosphere. When the jet stream dips to the south in the Northern Hemisphere, it creates a trough of low pressure, allowing cold polar air masses to invade the United States. Simultaneously, southwesterly flow ahead of the trough at high levels of the atmosphere directs warmer, moist air from the Gulf of Mexico over the low-level cold air. This overriding pattern of warm air over cold can cause steady precipitation over a wide area for as long as 12 to 24 hours at a time. If low level air is cold enough, the precipitation can fall in frozen form. It's not an uncommon winter weather pattern for southwesterly winds aloft to spread clouds all the way from the Gulf Coast to the Great Lakes, with rain falling across much of the south and wintry precipitation glazing areas farther north. ____________________________________________________________________________________________________ Temp. Inversions relation to ham radio: ( www.tpub.com ) Under normal atmospheric conditions, the warmest air is found near the surface of the Earth. The air gradually becomes cooler as altitude increases. At times, however, an unusual situation develops in which layers of warm air are formed above layers of cool air. This condition is known as TEMPERATURE INVERSION. These temperature inversions cause channels, or ducts, of cool air to be sandwiched between the surface of the Earth and a layer of warm air, or between two layers of warm air. If a transmitting antenna extends into such a duct of cool air, or if the radio wave enters the duct at a very low angle of incidence, vhf and uhf transmissions may be propagated far beyond normal line-of-sight distances. When ducts are present as a result of temperature inversions, good reception of vhf and uhf television signals from a station located hundreds of miles away is not unusual. These long distances are possible because of the different densities and refractive qualities of warm and cool air. The sudden change in density when a radio wave enters the warm air above a duct causes the wave to be refracted back toward Earth. When the wave strikes the Earth or a warm layer below the duct, it is again reflected or refracted upward and proceeds on through the duct with a multiple-hop type of action.