The Human METAR

Another storm is expected to dump another 6 inches or more of snow late tonight into early tomorrow morning. Here is the NWS output this morning:

.LOW PRESSURE OVER THE GREAT LAKES STATES TODAY WILL MOVE EAST...
AS A SECONDARY LOW PRESSURE SYSTEM DEVELOPS OVER SOUTHERN NEW
ENGLAND THURSDAY MORNING. THE SECONDARY LOW WILL MOVE SLOWLY
NORTHEAST ALONG THE MAINE COAST THURSDAY AFTERNOON. THE
COMBINATION OF SYSTEMS WILL BRING A SIGNIFICANT WINTER STORM TO
THE REGION...BEGINNING LATE THIS AFTERNOON IN NEW HAMPSHIRE AND
SPREADING EAST INTO MAINE THIS EVENING.

SNOW WILL MOVE INTO SOUTHWESTERN NEW HAMPSHIRE LATE THIS
AFTERNOON...SPREADING NORTHEAST INTO CENTRAL MAINE BY LATE THIS
EVENING. SNOW WILL INITIALLY BE LIGHT...BUT WILL PICK UP IN
INTENSITY AFTER MIDNIGHT. A BAND OF HEAVY SNOW IS EXPECTED TO MOVE
ACROSS THE REGION DURING THE EARLY MORNING HOURS AND INTO THE
MORNING COMMUTE ON THURSDAY.

ALONG THE COAST AND IN FAR SOUTHERN NEW HAMPSHIRE...SNOW WILL MIX WITH AND
EVENTUALLY CHANGE TO RAIN LATE TONIGHT AND THURSDAY MORNING. THIS
WILL HOLD DOWN SNOWFALL ACCUMULATIONS THERE.

THE LOW WILL STALL ACROSS MAINE ON THURSDAY AFTERNOON. THIS WILL
CAUSE LIGHT SNOW TO LINGER IN CENTRAL MAINE AND THE MOUNTAINS A
BIT LONGER...WITH SOME ADDITIONAL ACCUMULATION POSSIBLE THROUGH
THURSDAY NIGHT.

Because I am lazy, and don’t feel like competing, I am sending you to Norman’s page for a reliable forecast for Plymouth.

Another snow storm is expected to hit the New England area on Wednesday. The National Weather Service has issued a Winter Storm Watch in effect all day Wednesday. Luckily it isn’t strong as the past storms we have had, and its not going to last long, but it will still dump a few inches. Here is my forecast:

Tuesday Night – Clear skies becoming cloudy overnight. Winds light and from the north. Slight chance of snow showers late into the night with an overnight low of 7°F. Chance of precipitation is 30%. Little or snow accumulation possible.

Wednesday – Cloudy and snowy. High of 26°F with snow accumulations between 6-8 inches. Winds from the NE between 5 and 10 mph . Chance of precipitation 90%.

Wednesday Night – Cloudy with snow tapering off after midnight. Overnight low of 10°F. Northerly wind between 10 and 15 mph.  Nighttime snow accumulations of 2-3 inches. Chance of precipitation is 60%.

Thursday – Partly Cloudy skies. High of 29°F. Winds light and variable.

Forecasting overnight lows can be tricky in the winter months, because the presence of snow, calm winds, and clear skies can induce radiational cooling, where all of the infrared radiation leaves the earth and causes the temperature to drop drastically within the first few hundred meters. This inversion can be seen on morning soundings, such as the following sounding taken at KGYX on the morning of 1/2/09:

Near the surface, the temperature got as low as -11°C, but at just 500m up, the temperature was about 9 degrees warmer. This is a case of radiational cooling.

Well how can you forecast for this? Well there is a simple, semi-accurate way of doing it:

1. Find the coldest 850mb temp overnight (from Models)
2. Using a skew-t chart, start at the temperature at 850mb and come down to the surface moist adiabatically
3. Pick off the temperature and convert to Fahrenheit.
4. IF there are clear skies, subtract 10°F
5. IF there are clear skies AND calm winds, subtract another 5°F
6. IF there are clear skies, AND calm winds, AND fresh snow on the ground, subtract another 5°F
7. IF there are clear skies, calm winds, fresh snow, AND an arctic air mass, subtract another 5°F

As you can see, if there are clear skies, calm winds, fresh snow, and an arctic air mass (like we had last week), the temperature can easily drop 25°F! Man that would be really cold.

Example, using data for tonight’s overnight low

Using the GFS MOS, the coldest 850 temp we will see tonight is -19°C. Moving that down to the surface, that will give us a surface temp of -8°C, or 17.6°F.

Will there be clear skies?: YES! 700mb RH is low, and there is no strong troughs or fronts heading our way, so it looks like there will be clear skies. 17.6 – 10 = 7.6°F

Will the winds be calm?: This one is tricky, becuase some areas of Plymouth may see some winds (downtown, on campus) but some areas could be calm (Baker River Valley). I am going to say however that any winds will die down late into the night, helping to lower the temperature. 7.6 – 5 = 2.6°F

Is there fresh snow?: Maybe, what is the exact definition of fresh snow. The last time we had significant snow fall was Monday. Few days ago, but Im going to say it’s still fresh. 2.6 – 5 = -2.4°F

Arctic Air Mass?: NO! The 850mb temps are above -30°C. I don’t consider that arctic

So after all that im going to round up and say that tonight’s low will be -2°F. Let’s see how this verifies tomorrow

As if i hasn’t gotten cold enough, temperatures are going to fall to single digits, if not below zero temperatures on Thursday and Friday.

Current 850mb analysis shows an upper level trough sweeping through the new england area tonight, which will give us about an inch or 2 of snow. However once it passes through, strong cold air advection upstream of the trough will help to bring in 850mb temperatures around -20C on Thursday, and -25°C on Friday.

Now I’m all for cold weather, but this is possibly too cold. The NWS in Gray Maine has issued this statement:

ARCTIC AIR ARRIVES ON WEDNESDAY AND WILL BRING THE COLDEST WEATHER
OF THE SEASON THUS FAR. GUSTY NORTHWEST WINDS ON WEDNESDAY WILL
PRODUCE WIND CHILLS OF 15 TO 25 BELOW IN THE MOUNTAINS AND
FOOTHILLS…AND BETWEEN ZERO AND 15 BELOW ELSEWHERE. WHILE WINDS
WILL DIMINISH BY WEDNESDAY EVENING…THE INTENSE COLD WILL LINGER INTO
SATURDAY BEFORE EASING. ACTUAL AIR TEMPERATURES WILL STRUGGLE TO
REACH THE SINGLE NUMBERS AND TEENS BY DAY…WITH OVERNIGHT LOWS IN
THE SINGLE NUMBERS AND TEENS BELOW ZERO.

48-72 hr Tempreature Forecast

Wednesday: 14°F

WednesdayNight: -1°F

Thursday : 8°F

Thursday Night: -10°F

Not only will I be trying to forecast some more, I also plan on verifying my forecasts to see how I do. However it’s still going to be mind-numbing cold no matter what, so BUNDLE UP!