Early Snow in Pennsylvania and New York

October 15th, 2009 |


Snow is widespread today from central Pennsylvania northward into central New York. Accumulating snow only rarely falls this early in the season in that part of the country, and at many stations this is the earliest measurable snow on record. The above image is an enhanced 11-micron image from GOES-12 with surface observations (in white) and buoy observations (in black) superimposed. (Click image to enlarge)

One reason for snow’s rarity can be viewed in the sea surface temperatures plotted in black — ocean surface temperatures are still in the low 60s off the coast of New Jersey. That source of relative warmth can have a powerful effect if the wind is from the east, and it can be fatal to a snowfall.

Much of the moisture from this storm, however, is not coming from the east, but from the west and southwest, as suggested in this plot of total precipitable water — plotted as a percentage of normal — derived from AMSU and SSM/I instruments on the NOAA polar orbiters. (See image here; note the axis of high percentages along the spine of the Appalachians).

A vital feature for the production of snow is the presence of ice crystals within a mixed-phase cloud. When that happens, the Bergeron-Findeisen process allows ice crystals to grow at the expense of water droplets, and these ice crystals can then fall towards the surface, either maintaining their integrity as snow all the way down or melting to rain drops. If ice crystals are not present, cloud droplets will grow via Collision and Coalescence, and a drizzle or light rain is more likely. Cloud types derived from different channels on MODIS (image here, valid at 1534 UTC on 15 October) and from AVHRR (image here, valid at 1721 UTC 15 October). Both images suggest that the presence of ice crystals for the seeder-feeder mechanism might be limited if winds at cloud level are westerly. Should that happen, the snow of this afternoon would become light rain or drizzle tonight. MODIS cloud phase from 1710 UTC of 15 October, however, which has a swath farther to the west, does show more ice clouds upstream of Pennsylvania and New York, an observation that suggests snow may continue, if surface and near-surface temperatures remain cool enough.

(Updated, 16 October: Some snow totals as of 1200 UTC 16 October: 4.7″ in State College, 3.2″ in Philipsburg, 2.4″ in Altoona, 5.9″ in Wellsboro)

Pacific Jet

October 12th, 2009 |


Columbus Day is the Anniversary of a truly historic storm in the Pacific Northwest. On October 12, 1962, one of the most intense storms on record caused wind gusts exceeding hurricane force over a broad swath of coastal Washington and Oregon. Now, 47 years later, a weaker but still potent storm is poised to move onshore.

Water vapor imagery (above) shows the characteristic signal of a strong jet, with a dark band, indicating sinking and drying (and warmer brightness temperatures because the satellite sensor is seeing farther down into the atmosphere), on the poleward side of the strongest winds. Aircraft wind observations (plotted in red) show numerous observations of wind speeds exceeding 150 knots, and the GFS analysis 6-hr forecast valid at 1200 UTC this morning (the time of the water vapor image) shows a 160+ knot jet on the 345 K isentropic surface.

The big storm of 1962 could be easily linked to a tropical cyclone that moved north just west of the Dateline. Is the jet now present in the Pacific linked to Typhoon Melor, which storm was off the coast of Japan last week? This animation of enhanced 11-micron imagery over the Pacific Ocean suggests that it is. Certainly the moisture from the tropical system is part of the jet; careful inspection of the imagery suggests that the mid-level vorticity from Melor is also involved in this jet.

Typhoon Melor: Is it generating a PRE?

October 6th, 2009 |

MTSAT IR image

Tropical Systems can occasionally be accompanied by Predecessor Rainfall Events (PREs). This band of heavy rainfall is not associated with the spiral bands of the tropical system, but rather with an interaction with a mid-latitude jet that exists Poleward of the tropical feature. Some notable PRE-producing tropical systems in the United States include Hurricane Ike from 2008 and Hurricane Floyd from 1994. Accumulated rainfall patterns from Ike and from Floyd show very heavy rains far removed from the landfall; in Ike’s case, over northwestern Indiana, and in Floyd’s case over Connecticut and New York. In both cases, PREs have been identified as a likely rain producer.

Typhoon Melor has been approaching the northwestern corner of the tropical Pacific Ocean over the past several days. At the same time, a ribbon of moist air, as denoted by high Precipitable Water values diagnosed from MIMIC, extends southwestward from Japan towards the straight of Luzon (Note also in the Precipitable Water loop the presence of former Typhoon — now Tropical Storm Parma meandering within the straight of Luzon as well.

The 11-micron window channel imagery show a general blossoming of cold cloud tops in and around the ribbon air with high precipitable water as typhoon Melor approaches (Link here). Indeed, the last image in the loop, shown above in this post, bears a resemblance to the enhanced 11-micron GOES-8 image from landfall of Floyd.

Is there evidence of a jet poleward of Melor that would support the development of the PRE? Consider the enhanced water vapor image from MTSAT below. The large gradient in brightness temperature — very warm values in and around Korea and the South China Sea northeastward to the Sea of Japan, and very cold temperatures to the east, suggest the presence of a strong jet. Plots of 300-hPa wind speeds confirm that; note the speeds exceeding 150 knots at Sapporo and at Nakashibetsu on the island of Hokkaido! The position of this jet is such that the left entrance region is supporting upward motion to help support heavy rain in a very moisture-rich atmosphere. (The GFS 00-h analysis at 1200 UTC 6 October shows this jet extending far out into the Pacific Ocean).


MTSAT water vapor image + 300 hPa rawinsonde wind speeds

MTSAT atmospheric motion vectors or “satellite winds”  (calculated by tracking satellite image features on 3 consecutive images) actually showed that wind speeds along the jet stream axis over the western Pacific Ocean were as high as 211 knots at the 218 mb level (below).

MTSAT water vapor image + MTSAT winds

MTSAT water vapor image + MTSAT winds

(Added: TRMM measurements of rainfall with Melor are here).

Tropical Storm Grace

October 5th, 2009 |

GOES-12 visible image

Late on October 4th, the weather system in the far northeast Atlantic acquired sufficient tropical characteristics to be classified as a Tropical Storm, and Grace was named. The visible image from GOES-12 above shows the counterclockwise swirl of clouds. GOES-12 is over the Equator at 75 degrees W Longitude, and Tropical Storm Grace at the time of the image above was at 45 degrees North latitude and 16 degrees W Longitude; consequently, the view angle is very oblique. Indeed, the visible image shows a convective spiral band that lies beneath the cirrus shield that covers the system. Note that no overshooting tops penetrate the cirrus overcast over the tropical system. The system sits over sea surface temperatures near 70 degrees Fahrenheit (see the Sea Surface Temperature analysis here, and those temperatures are yielding insufficient CAPEs to produce overshooting tops.

Grace developed underneath a decaying upper-level low. The low was able to draw north modestly high values of precipitable water, as shown in the MIMIC analysis here. Grace is associated with the very small region of enhanced precipitable water that is at 40 N, 20 W at the start of the loop, then moving northeastward towards Ireland.

A comparison of Terra MODIS visible and 11.0 µm IR images (below) showed that Grace exhibited a fairly well-defined banded structure and some semblance of an eye at 11:40 UTC.

MODIS visible and IR images

MODIS visible and IR images

(Added: Jesse Ferrell at AccuWeather notes that Grace was almost the farthest-east forming tropical system on record! Link).

(Added, 6 October: Grace merged with/was absorbed by a front southwest of Ireland late in the day on the 5th.) AMSU microwave data from early on the day on the 5th clearly show a warm core to the system, one of the hallmarks of a tropical Storm. For example, data from the AMSU-A instrument in NOAA-18 at 0413 UTC on 5 October show a region of warmth at 550 hPa (Channel 5), at 350 hPa (Channel 6) and at 200 hPa (Channel 7); the 89-GHz channel on AMSU-B also shows warmth at the center of the storm. These warm signals were critical in determining that system was tropical in nature. The warmth persisted; AMSU-A data from NOAA-19 at 1406 UTC on the 5th also showed a warm core at 550 hPa (Channel 5), at 350 hPa (Channel 6) and at 200 hPa (Channel 7), as well as in the 89-GHz channel on AMSU-B. (More imagery is available here).