December 1999 5-1 Section 5
SURFACE ANALYSIS CHART The surface analysis chart is a computer-generated chart, with frontal analysis by HPC forecasters, transmitted every 3 hours covering the contiguous 48 states and adjacent areas. Figure 5-1 is a surface analysis chart, and Figure 5-2 illustrates the symbols depicting fronts and pressure centers.
VALID TIME Valid time of the chart corresponds to the time of the plotted observations. A date-time group in Universal Coordinated Time (UTC) tells the user when conditions portrayed on the chart occurred.
ISOBARS Isobars are solid lines depicting the sea-level pressure pattern and are usually spaced at intervals of 4 millibar (mb), or hectoPascals (hPa) in metric units (1 millibar = 1 hectoPascal). Each isobar is labeled.
For example, 1032 signifies 1,032.0 mb (hPa); 1000 signifies 1,000.0 mb (hPa); and 992 signifies 992.0 mb (hPa).
PRESSURE SYSTEMS The letter “L” denotes a low pressure center, and the letter “H” denotes a high pressure center. The pressure of each center is indicated by a three- or four-digit number that is the central pressure in mb (hPa).
FRONTS The analysis shows positions and types of fronts by the symbols in Figure 5-2. The symbols on the front indicate the type of front and point in the direction toward which the front is moving. If the front has arrowhead-shaped symbols, it is a cold front. If the front has half-moon symbols, it is a warm front.
A three-digit number near a front classifies it as to type (see Table 5-1), intensity (see Table 5-2), and character (see Table 5-3). A bracket ( [ or ] ) before or after the number “points” to the front to which the number refers. For example, in Figure 5-1, the front extends from eastern Montana into central North Dakota south through South Dakota and Nebraska into northwestern Kansas. The front is labeled “027” which means a quasi-stationary front (“0” from Table 5-1); weak, little, or no change (“2” from Table 5-
2); and with waves (“7” from Table 5-3).
Two short lines across a front indicate a change in classification. In figure 5-1, note that two lines cross the front in central Montana (adjacent to the Low). To the left of the Low the front is numbered “450” which is a cold front; moderate, little, or no change; and no specification. The front to the right of the Low is numbered “027” which is a quasi-stationary front; weak, little, or no change; and with waves.
TROUGHS AND RIDGES A trough of low pressure with significant weather will be depicted as a thick, dashed line running through the center of the trough and identified with the word “TROF.” The symbol for a ridge of high pressure is very rarely, if at all, depicted (Figure 5-2).
December 1999 5-2
OTHER INFORMATION The observations from a number of stations are plotted on the chart to aid in analyzing and interpreting the surface weather features. These plotted observations are referred to as station models. There are two primary types of station models plotted on the chart. Those with a round station symbol are observations taken by observers. The locations with a square station symbol indicate the sky cover was determined by an automated system. Other plotting models that appear over water on the chart are data from ships, buoys, and offshore oil platforms. Figure 5-3 is an example of a station model that shows where the weather information is plotted. Figures 5-4 through Figure 5-7 help explain the decoding of the station model.
An outflow boundary will be depicted as a thick, dashed line with the word “OUTBNDY.”
A dry line will be depicted as a line with unshaded pips or a through symbol. It will also be identified with the words “DRY LINE.”
A legend is printed on each chart stating its name, valid date and valid time.
USING THE CHART The surface analysis chart provides a ready means of locating pressure systems and fronts. It also gives an overview of winds, temperatures, and dew point temperatures at chart time. When using the chart, keep in mind that weather moves and conditions change. Using the surface analysis chart in conjunction with other information gives a more complete weather picture.
December 1999 550110 25) 157 1
58. 154 1022 00Z THU 08 JAN 1998 HPC SURFACE ANALYSIS NDA/ NS INCEP CASHINGTON SQUARE SKY-COVER STOLS A ME AUTOMATIC VOLCANIC
December 1999 Color Symbol Description Blue High Pressure Center Red Low Pressure Center Blue Cold Front Red Warm Front Red/Blue Stationary Front Purple Occluded Front Blue Cold Frontogenesis Red Warm Frontogenesis Red/Blue Stationary Frontogenesis Blue Cold Frontolysis Red Warm Frontolysis Red/Blue Stationary Frontolysis Purple Occluded Frontolysis Purple •• Squall Line Brown Dryline Brown Trough Yellow Ridge Figure 5-2. Symbols on Surface Analysis Chart.
5-4
December 1999
HIGH CLOUD TYPE
TOTAL SKY COVER
MIDDLE CLOUD TYPE
WIND SPEED , SEA-LEVEL PRESSURE
WIND DIRECTION PRESSURE CHANGE IN 147 • PAST 3 HOURS TEMPERATURE 34 PRESENT WEATHER * ** 28 / • PRESSURE TENDENCY DEW POINT → 32 LOW CLOUD TYPE .45 - 6-HOUR PRECIPITATION
1. Total sky cover: Overcast.
2. Temperature: 34 degrees F, Dew Point: 32 degrees F.
3. Wind: From the northwest at 20 knots (relative to true north).
Examples of wind direction and speed
CALM NORTHEAST SOUTHWEST NORTH AT WEST AT SOUTH AT AT 5 KNOTS AT 10 KNOTS 15 KNOTS 50 KNOTS 60 KNOTS
4. Present Weather: Continuous light snow.
5. Predominate low, middle, high cloud reported: Strato fractus or cumulus fractus of bad weather, altocumulus in patches, and dense cirrus.
6. Sea-level pressure: 1,014.7 millibars (mbs).
NOTE: Pressure is always shown in three digits to nearest tenth of an mb. For 1,000 mbs or greater, prefix a "10" to the three digits. For less than 1,000 mbs, prefix a "9" to the three digits.
7. Pressure change in the past 3 hours: Increased steadily or unsteadily by 2.8 mbs. The actual change is in tenths of a mb.
8. 6 - hour precipitation in hundredths of an inch: 45 hundredths of an inch.
Figure 5-3. Station Model and Explanation.
5-5
December 1999 5-6 Table 5-1. Type of Front Code Figures Descriptions 0 Quasi-stationary at surface 2 Warm front at surface 4 Cold front at surface 6 Occlusion 7 Instability line Table 5-2. Intensity of Front Code Figures Descriptions 0 No specification 1 Weak, decreasing 2 Weak, little, or no change 3 Weak, increasing 4 Moderate, decreasing 5 Moderate, little, or no change 6 Moderate, increasing 7 Strong, decreasing 8 Strong, little, or no change 9 Strong, increasing Table 5-3. Character of Front Code Figures Descriptions 0 No specification 5 Forming or existence expected 6 Quasi-stationary 7 With waves 8 Diffuse
December 1999 5-7 Clear (0/8) Breaks in overcast FEW (<1/8 - 2/8) OVC (8/8) SCT (3/8 - 4/8) Total sky obscuration (8/8) BKN (5/8 - 7/8) Missing cloud (or sky cover) observation or partial obscuration Figure 5-4. Sky Cover Symbols.
December 1999 5-8 Primary Additional Requirements Requirements Same Atmospheric pressure now same as 3 hours ago.
Increasing, then steady; or Increasing, then increasing more slowly Increasing, then decreasing Decreasing; or steady, then increasing; or Increasing, then increasing more rapidly Higher Atmospheric pressure now higher than 3 hours ago.
Lower Atmospheric pressure now lower than 3 hours ago.
Increasing;
steadily or unsteadily Increasing, then decreasing Steady Decreasing, then increasing Decreasing, then increasing Decreasing, then steady; or Decreasing, then decreasing more slowly Steady; or increasing, then decreasing; or Decreasing, then decreasing more rapidly Code Figure 0 1 2 3 0 4 5 5 6 7 8 Figure 5-5. Pressure Tendencies.
Description of Characteristic Graphic Decreasing;
steadily or unsteadily
December 1999 or at station =¡ without precipitation) luring past hour, but NOT at time of observation. Slight shower(s) of hail, rain and snow mixed, ht.
out NOT mixed.
for de. *** ahservation * * ›bservation hour:.
00 10 20 30 40 50 60 70 80 90
December 1999 DESCRIPTION (Abridged from W.M.O. Code) Filaments of Ci, orimares tails, scattered and not increasing Dense Ci in patches or twisted sheaves, usually not increasing, sometimes like remains of Cb; or towers tufts Dense Ci, often anvil shaped derived from or associated Cb Ci, often hook shaped gradually spreading over the sky and usually thickening as a whole Ci and Cs, often in converging bands or Cs alone; generally overspreading and growing denser; the continuous layer not reaching 45 altitude Ci and Cs, often in converging bands or Cs alone; generally overspreading and growing denser; the continuous layer exceeding 45 altitude Veil of Cs covering the entire sky Cs not increasing and not covering the entire sky Cc alone or Cc with some Ci or Cs but the Cc being the main cirroform cloud 2 DESCRIPTION (Abridged from W.M.O. Code) Thin As (most of cloud layer is semitransparent) Thick As, greater part sufficiently dense to hide sun (or moon), or Ns Thin Ac, mostly semitransparent; cloud elements not changing much at a single level Thin Ac in patches: cloud elements continually changing and/or occurring at more than one level Thin Ac in bands or in a layer gradually spreading over sky and usually thickening as a whole Ac formed by the spreading out of Cu Double-layered Ac, or a thick laver of Ac, not increasing: or Ac with As and/or Ns Ac in the form of Cu- shaped tufts or Ac with turrets Ac of chaotic sky, usually at different levels; patches of dense Ci are usually present Figure 5-7. Cloud Symbols DESCRIPTION (Abridged from W.M.O. Code) Cu, fair weather, little vertical development and flattened Cu, considerable development, towering with or without other Cu or Sc bases at same level Cb with tops lacking clearcut outlines, but distinctly not cirroform or anvil shaped; with or without Cu, Sc, or St Sc formed by spreading out of Cu; Cu often present also Sc not formed by spreading out of Cu St or F's or both, but no Fs of bad weather F's and/or Fc of bad weather (scud) Cu and Sc (not formed by spreading out of Cu) with bases at different levels Cb having a clearly fibrous (cirroform) top, often anvil shaped, with or without Cu, Sc, St, or scud CLOUD ABBREVIATION St or Fs - Stratus or Fractostratus Ci - Cirrus Cs - Cirrostratus Cc - Cirrocumulus Ac - Altocumulus As - Altostratus Sc - Stratocumulus Ns - Nimbostratus Cu or Fc - Cumulus or Fractocumulus Cb - Cumulonimbus 5-10
December 1999 6-1 Section 6
WEATHER DEPICTION CHART The weather depiction chart, Figure 6-3, is computer-generated (with human frontal analysis) from METAR reports. The weather depiction chart gives a broad overview of the observed flying category conditions at the valid time of the chart. This chart begins at 01Z each day, is transmitted at 3-hours intervals, and is valid at the time of the plotted data.
PLOTTED DATA Observations reported by both manual and automated observation locations provide the data for the chart.
The right bracket ( ] ) indicates the present weather information was obtained by an automated system only. The plotted data for each station are total sky cover, cloud height or ceiling, weather and obstructions to vision, and visibility. If the stations on the chart are crowded together, the weather, visibility, and cloud height may be moved up to 90 degrees around the station for better legibility. When reports are frequently updated, as at some automatic stations (every 20 minutes) or when the weather changes significantly, the observation used is the latest METAR received instead of using the one closest to the stated analysis time.
TOTAL SKY COVER The amount of sky cover is shown by the station circle shaded as in Figure 6-1.
Clear (0/8) Breaks in overcast FEW (<1/8 - 2/8) OVC (8/8) SCT (3/8 - 4/8) Total sky obscuration (8/8) BKN (5/8 - 7/8) Missing cloud (or sky cover) observation or partial obscuration Figure 6-1. Total Sky Cover.
December 1999 6-2
CLOUD HEIGHT Cloud height above ground level (AGL) is entered under the station circle in hundreds of feet; the same as coded in a METAR report. If total sky cover at a station is scattered, the cloud height entered is the base of the lowest scattered cloud layer. If total sky cover is broken or greater at a station, the cloud height entered is the lowest broken or overcast cloud layer. A totally obscured sky is shown by the sky cover symbol “X” and is accompanied by the height entry of the obscuration (vertical visibility into the obscuration). A partially obscured sky without a cloud layer above, however, is not recognized by the computer program reading the METAR report. It cannot differentiate between a partial obscuration and a missing observation. Therefore, the computer program will enter an “M” in the sky cover circle for either occurrence. Consequently, the user will not know if the observation is missing or a partial obscuration is present. To obtain the most accurate information, the user must consult the METAR report for that specific station. A partially obscured sky with clouds above will have a cloud height entry for the cloud layer, but there will be no entry to indicate that there is a partial obscuration at the surface. So once again the user must consult the METAR report to obtain the most accurate information.
WEATHER AND OBSTRUCTIONS TO VISIBILITY Weather and obstructions to visibility symbols are entered to the left of the station circle. Figure 5-6 explains most of the symbols used. When several types of weather and/or obstructions to visibility are reported at a station, the first one reported in the METAR would usually be the highest coded number in Figure 5-6. Also, for some stations that are not ordinarily plotted, the weather symbol is plotted only if the weather is significant, such as a thunderstorm.
VISIBILITY When visibility is 5 miles or less, it is entered to the left of the weather or obstructions to vision symbol.
Visibility is entered in statute miles and fractions of a mile.