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February 2, 2010

NWS boosts advisory to 3 to 6 inches

NOAAOh my... The overnight storm that was to deliver 1 to 3 inches Tuesday into Wednesday has been boosted to a 3-to-6-incher. The National Weather Service has upped the ante to a Winter Storm Warning, which means we have the potential for more than 4 inches of the White Death.

The Warnings have been posted along a swath (pink on the map) from Northern Virginia  across DC to Central Maryland, including Baltimore and the surrounding counties from Montgomery and PG to Arundel, Howard, Frederick, Carroll, Baltimore and Harford.

They have also swept Calvert and St. Mary's into the old Winter Weather Advisory, for 1 to 3 inches instead of's snow map (right) looks like it's been overtaken by events.  

So what's going on here? Apparently, better agreement among the forecast models, and therefore greater confidence in the chances for snow, heavy at times through DC and Baltimore, as these weather systems converge on the region tonight.

The WSW opens at 5 p.m. for Baltimore, with snow starting here between 5 and 7 p.m. Will that mean a rush hour snowfall? Maybe. Should folks bug out and head for home early? Drop me a comment when you start to see snow falling where you are.

Posted by Frank Roylance at 1:14 PM | | Comments (31)
Categories: Forecasts


Bug out and head home early? Heck, I'm ready to bug out and head to Florida until May 1. I'm sick of this winter.

I hope this forecast is accurate. I'm following Mr. Foot from now on.

FR: Careful. Nobody's right all the time.

Everybody should have bugged out and left at noon. Wait... that's advice for summer!

You're all wusses...It's only 6 inches of snow. Deal w/it and make it in time on to work tomorrow.

same here, NotableM. Their predictions seem more robust and show you why they are forecasting like they are. Quickly becoming a big fan Foot's Forecast ever since the Dec 18 storm.

Looks like Mr Foot's crew has nailed another one. I hope they are wrong about the BIG one this coming weekend though. Their early predictions have my back hurting already in anticipation of all the shoveling I'll have to do!

Is it snowing yet in Columbia?

I'm loving Mr. Foot's forecast for the weekend. I better go buy more chocolate! I think I'll make sure I'm ahead of my deadlines at work as well...

Maybe they will actually get this one right, unlike the "less than one inch" prediction from the weekend.

Being a weatherman must be great. No accountability for being wrong.

Despite radar indications to the contrary, precipitation is NOT YET reaching the ground here in west-central MoCo.

First flakes just started falling in west-central MoCo.

Actually, yesterday Foot was saying 2.5 for BWI. Now for this weekend, Foot is saying maybe 24 inches.

It started snowing in Frederick @ 4:30 or so...flurries @ 5:15 it is "plain ol' snow!"

please stay off the road when they are covered in snow unless you have 4x4. how many cars got recked and people got hurt last saturday because they just had to go out even though their cars can barely stay on the road

Mr. Foot is no more reliable than anyone else. I do like his team's explanations though.

Snowing here now and laying in Hanover, PA. Frank, would we fall under that 3-6" region?

Uh, Mike, cars got wrecked and people got hurt last weekend because they weren't calling for any snow. The next thing you know, five inches get dumped, and people are out and about because the weather man says that nothing is going to happen. Some accurate forecasts would be nice.

I am one of the nuts that love snow. Common sense goes so far when driving in it. If you are afraid to drive, don't. If you are afraid and driving @15 to 20 mph please put the phone down. And remember 4X4 does not help on ICE.
The forecasters do their best with the info on hand, When I was a kid, you looked out the window...

FR: Agreed. The forecast from the night before doesn't matter much ... If there is snow on the roads and more falling, that's all a driver needs to know. BTW, it's snowing on the Weatherdeck in Cockeysville at 6:30 pm.

It started snowing (slight flurries, really) here in Owings Mills right around 5:30, as I was heading out to pick up some groceries. By 6, it was sticking, and now (7) it's snowing pretty hard.

Snowing in Essex too.For about an half an hour.

Hi from the "rainy" south. Being raised in Baltimore I kind of miss the snow. Well, mayb not. Have fun and remeber spring is right around the corner. Hi Judy!

I love reading stories about the cold snowy weather in Maryland. Especially since I moved to Hawaii. ALOHA!

Since no one answered my question about why nor'esters track up the coast so often I did some research and found the answer below

SHW Ch. 10 - Extratropical Cyclones Forming Along the East & Gulf Coasts

Coastal Extratropical Cyclones


Preferentially form along the central East Coast (near Cape Hatteras, NC) and just off the Gulf Coast (centered near the TX-LA border); can, and do, form at other points along each coast; Figure 10.2

East and Gulf coast cyclones are often more intense that their Rocky Mountain counterparts, due to the following four major factors:

1) Latent heat released during condensation in the clouds contributes more to storm intensification; local moisture source in the warm Gulf of Mexico and in the warm Gulf Stream current.

2) Sensible heat from the ocean (gulf) surface acts to heat the atmosphere, contributing more to storm intensification; heat energy transferred directly from water to air through conduction.

3) Strong thermal contrasts between the ocean (gulf) and land enhance and maintain a sharp thermal boundary (i.e., baroclinic zone) along the coastline; most pronounced in winter.

4) There is often more than one jetstreak acting to create upper-level divergence; the jetstreaks are found within two separate jetstreams (polar and subtropical) and their "phasing" creates the strongest storms.

East Coast Cyclones


East coast cyclones typically develop after an earlier (primary) cyclone developed to the lee of the Rockies and tracked near or through the Great Lakes or Ohio Valley regions and into Canada, thus setting up the necessary environment for the secondary low to form.

Environment Prior to Development:

Figure 10.3A: The primary low moves cold air southeastward across the continent behind its cold front, spilling over the Appalachians and reaching the east coast, thus creating the necessary baroclinic zone.

Figure 10.3D: The polar jetstream is now flowing across the central Atlantic coast, with a jetstreak (labeled J1 in the figure) on the west side of the trough, approaching the east coast.

Figure 10.4: If high pressure is located over New England or southeastern Canada, it will set up an easterly flow of warmer, maritime air into the east coast, thus creating a "coastal front" at the leading edge of the baroclinic zone; this also creates "cold air damming".

The "coastal front" becomes the boundary on which the secondary low will develop.

In the development of a strong east coast cyclone, a subtropical jet stream will merge with the polar jet off the coast coast and provide a second jetstreak (labeled J2 in Figure 10.3D), that acts with J1 to enhance the upper-level divergence.

Initial Development:

Figure 10.3E: The jetstreak on the west side of the polar jet (J1) migrates toward and into the base of the trough; the left-exit region of J1, in addition to the change in curvature east of the trough, maximizes upper-level divergence on the east side of the trough over the "coastal front."

Figure 10.3E: The jetstreak in the higher altitude subtropical jet (J2) also contributes to the divergence region, as its left-exit region superimposes with J1.

Figure 10.3B: A surface low pressure will rapidly form and develop directly underneath the region of maximum upper-level divergence, enhanced by latent heat release and sensible heat transfer from the ocean by the action of surface wind circulations.

A "bomb cyclone" has a central pressure drop of 24 mb/24 hours, normalized at 60° N Latitude.

(Continued: SHW Ch. 10 - Extratropical Cyclones Forming Along the East & Gulf Coasts)

Storm Evolution:

Figure 10.3C: East coast cyclones (called Nor'easters) track northeast up along the coast, typically reaching their most intense stage 24 to 48 hours after initial development.

Dry air descends from the upper troposphere (Figure 10.5) to create the prominent "dry slot".

"Conveyor Belt Model" (discussed in Ahrens Ch. 13) describes the air mass flow into and around the low.

Figure 10.3F: Sometimes, the polar jetstreak (J1) and subtropical jetstreak (J2) merge ("phase"), creating a structure to the northwest of the cyclone center, similar to the "trowal" associated with the strong Rocky Mountain cyclones; can result in the greatest blizzards.

The upper-level low continually deepens until if becomes a "cut-off low"; the surface low and upper low align vertically to create a deep vortex.

Figure 10.3F: As the merged jetstreak propagates out of the trough and into the downstream ridge, and the flow around the "cut-off low" consists of counterclockwise flow (i.e., no longer any curvature change), the upper-level divergence ceases and the surface low "fills" (i.e., weakens).

Surface friction aids in the "filling" process, as it causes air to flow into the low, but is slower over water than land due to reduced friction over water, thus oceanic cyclones can remain strong for several days as they move up along and off the coast.

Gulf Coast Cyclones


Gulf coast cyclones develop most frequently during years when the subtropical jet is a persistent strong feature in the upper troposphere over northern Mexico and the Gulf of Mexico (such as during "El Nino").

Gulf coast cyclones typically follow one of two tracks (Figure 10.2), either first along the Gulf coast and then northeast up the Atlantic Seaboard (east of the Appalachians), or from the Gulf coast, inland along the Mississippi and Ohio River Valleys (west of the Appalachians).

1: East Coast Storm Track


Figure 10.6A: Cold (or Arctic) front moves across the U.S. And arrives at the East and Gulf coasts, thereby setting up a strong barolclinic zone between the cold air and the warmer waters.

Figure 10.6E: A large upper-level trough is often present over the entire eastern U.S., usually as a result of an earlier Rocky Mountain cyclone; there is usually a jetstreak (labeled J2 in this figure) to the west of the trough axis.

Figure 10.6E: A subtropical jet flows from off the Pacific Ocean, across Mexico and over the Gulf coast; the subtropical jetstreak (labeled J1 in this figure) is the one that triggers storm formation.

Figure 10.6B & 10.6F: the Gulf coast surface low develops in response to the left-exit region jetstreak effects of J1; this begins the low-level wind circulation pattern that drives warm air northward and cold air southward, creating "barolclinic instability"; latent heat and sensible heat transfers enhance this development process.

Note: as the cold front is forced south over the Gulf of Mexico, it can trigger a line of strong T-storms.

Figure 10.6C: the surface low will move eastward to remain under the divergent left-exit region of J1.

Figure 10.6G: sometimes the approach of J2 can superimpose its left-exit region effects, leading to explosive deepening; this is not a necessary factor in development, but will affect the magnitude of the upper-level divergence and hence, the ultimate lowest central pressure of the cyclone.

Gulf coast cyclones will continue to intensify as long as the upper-level divergence associated with the jetstreaks, the changes in flow curvature and latent heat release in the clouds (due to precip) all together EXCEED the convergence at the surface due to friction.

Figure 10.6D: the fully matured cyclone has developed the "comma-shaped" cloud pattern associated with the "trowal-like" structure to the northwest of the low (where the heaviest precip falls) and the "dry slot" from the subsiding upper tropospheric polar jet; the surface system is occluded at this point.

Figure 10.6H: the two jetstreaks, J2 & J1, have merged to the east of the cyclone; "cut-off low" forms.

The dissipation stage is the same as with other previously discussed cyclone development.

(Continued: SHW Ch. 10 - Extratropical Cyclones Forming Along the East & Gulf Coasts)

2: Mississippi-Ohio River Valley Storm Track


Figure 10.7C: this track is more likely when the upper-level trough is farther west over the central U.S. Prior to formation, with the airflow more southerly over the eastern third of the U.S., the subtropical jet merges into the polar jet to the east of the trough axis.

Figure 10.7A: a surface low develops in response to the divergent left-exit region of the subtropical jetstreak (labeled J1 in this Figure).

Figure 10.7D: the single jetstreak, J1, remains over land, flowing northward over the eastern U.S.

Figure 10.7B: the surface low tracks to the north and east, remaining under the maximum upper-level divergence, passing between the Mississippi River Valley and Appalachians, up to the Ohio Valley; the heaviest precip typically occurs in advance of the warm front and in the "wrap-around" precip band extending to the northwest of the surface cyclone.

Note: strong southerly winds in the southeast sector of these storms transport warm, moist air from the Gulf of Mexico and over the Appalachians, which can lead to flash flooding.

The dissipation stage is the same as with other previously discussed cyclone development.

Forecasting and Assessing the Impact of Coastal Cyclones


Coastal cyclones affect the heavily populated corridor between Washington DC and Boston, MA.

A forecast of significant snowfall from Nor'easters sets into motion an expensive and disrupting chain of public actions designed to protect people and property (DOT snowplows to clear roads, extra police & paramedics for accidents and stranded people, school and business closings, closed airports or canceled flights, etc.).

Millions of dollars (spent or lost) are tied to the accuracy of storm forecasts!

Numerical Weather Prediction (NWP) models are getting better, but determining the exact location of the "rain/snow line" remains a problem for the coastal cities, due to current "resolution" (i.e., grid point spacing) of the models; a distance of only 20 km (~12 miles) can mean the difference of crippling heavy snow or just another rainy day.


Professor Gregory Zielinkski (University of Maine) proposed a winter storm classification scale (similar to the Saffir-Simpson Scale for hurricanes) from 1 to 5 that is based on: a) central low pressure of the cyclone; b) its 12-hour deepening rate; c) the maximum pressure gradient between the cyclone and its nearest high pressure. While this scale has the advantage of rating a storm in progress and disseminating the information to the public, it does not communicate information about expected snowfall.


Dr. Paul Kocin (TWC) and Dr. Louis W. Uccelini (NCEP Director) proposed a index called "Northeast Snowfall Impact Scale" (NESIS), which takes into account both the snowfall distribution and amounts, as well as the population distribution and density (based on the 2000 Census). The NESIS Index (from 1 to 5) classifies storms on their impact of snowfall on major population centers. While this index has the advantage of classifying storms on their impacts on society, the disadvantage is that it cannot be applied while the storm is in progress.

FR: This is why God created editors.



We're all doomed!

Oh... "the White Death" referred to a couple inches of snow.

Maybe all those perishables purchased for the long-haul weren't as necessary as I'd thought.

Are the global warming groupies still trying to use the old "weather and not climate" argument now?? We've already exceeded our annual seasonal snowfall by leaps and bounds! People really need to understand there are two sides to global warming and PLENTY of scientific evidence that refutes global warming theory....It's just too bad that this info. never makes it into any textbooks or mainstream media reports, thus making it impossible to have a rational debate on the topic. Thank You.

Mr. Getzel: Please just post a link next time. Thanks! :)

"FR: This is why God created editors."

HA! Indeed.

And to all those who think forecasters should be held 'accountable' — GROW UP!

Predicting the weather is so complex that the constantly changing variables and processes involved would easily bewilder the mental midgets who accuse without a clue. Those who make a living researching and predicting the weather for our benefit deserve some regard. They get it right far more often than not.

And as for the hack Roylance . . . I had the privilege of working with Frank for 3 years. He is one of the most intelligent, thoughtful and respectable professional journalists I know. But perhaps his greatest trait is that he cares enough to make every possible attempt to get it right.

Frank, thank you for your selfless efforts and continued service to the community.


FR: Thanks, Mom. Now turn off the computer.

Uh . . . Ted, could you go over that again, please?

Will somebody please MAKE SURE to answer Ted's question next time!


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About Frank Roylance
This site is the Maryland Weather archive. The current Maryland Weather blog can be found here.
Frank Roylance is a reporter for The Baltimore Sun. He came to Baltimore from New Bedford, Mass. in 1980 to join the old Evening Sun. He moved to the morning Sun when the papers merged in 1992, and has spent most of his time since covering science, including astronomy and the weather. One of The Baltimore Sun's first online Web logs, the Weather Blog debuted in October 2004. In June 2006 Frank also began writing comments on local weather and stargazing for The Baltimore Sun's print Weather Page. Frank also answers readers’ weather queries for the newspaper and the blog. Frank Roylance retired in October 2011. Maryland Weather is now being updated by members of The Baltimore Sun staff

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